Cohort Studies.

Cohort Studies

LEARNING OBJECTIVES Learner should be able to know at the end of this lecture What is a cohort study? What types of cohort studies are available? How to test the hypothesis by means of a cohort study?

Study designs cohort Case studies Case series co-relational
Study types Descriptive (formulation hypothesis) Individual based Case studies Case series Population based co-relational Analytical (testing hypothesis) Observational cohort Case-control Cross-sectional interventional RCT’s (III) Quazi experimental

WHAT IS A COHORT? Cohort is a group having a common characteristic
e.g. A smoker’s cohort means all are smokers in that group

PURPOSE OF COHORT STUDY
They try to prove the causal influence on the disease outcome in a forward direction from cause to the effect and in a comparative fashion

CHOICE OF STUDIES The choice of analytical design depends upon the feasibility, the setting and the requirements of the study. Some believe that case control study is the initial step for establishing causality later to be followed by cohort study. But case control technique is uniquely useful in certain circumstances Both the case control and cohort designs are having their own merits and demerits.

COHORT STUDIES While the biases is the main problem with the case-control studies, Follow up, attrition and ethical problems are the hurdles for the cohort study.

COHORT STUDY DESIGN In a COHORT STUDY, a group of individuals that is exposed to a risk factor (study group) is compared with a group of individuals not exposed to the risk factor (control group).

The researcher follows both groups over time
The Follow up period depends on the incubation period of disease Compares the occurrence of the disease related to the risk factor in the two groups Determines whether a greater proportion of those with the risk factor are affected The Incidence of the disease, the exposure risk, the disease risk and the population risks are measured directly

TYPES 1. Current cohort or prospective cohort 2. Retrospective cohort (study commenced at a point of time in the past unto the present) 3. Ambi-spective cohort (commenced in the past and extended into the future) It can be commenced in the past or present, can be carried onto the present or extended into the future according to the needs of the study. Sometimes, case control design may also be nested in a cohort design. Cohort design is flexible enough to suit the needs of both the study question and the investigator’s time and resources.

Prospective [Forward looking] study
Suspected cause Effect / disease Forward looking Now Future

Prospective Cohort Study
Figure 9.7 Time 1 / Future Time 0 / Now CHD NE NE E E CHD NE NE NE NE E NE CHD E NE NE CHD NE CHD NE E NE NE NE E NE E NE E NE NE CHD E E NE Time E Future future now Now Exposure is known and outcomes are explored

Figure : Retrospective cohort study
CHD NE NE NE E NE E NE CHD NE NE NE NE E NE NE CHD E NE NE CHD NE CHD NE E NE NE NE NE E E NE E NE NE NE E NE E CHD E E NE E E E Time Now Past Exposure is known and outcomes are explored

Ambi-spective Cohort Study
Figure 9.7 Time 1 / Future Time 0 / Now Past CHD NE NE E E CHD NE NE NE NE E NE CHD E NE NE CHD NE CHD NE E NE NE NE E NE E NE E NE NE CHD E E NE Time E Future future Now Past now Exposure is known and outcomes are explored

Methodology [selection of cohort]
Special group General population Comparison cohort (Not exposed) Study cohort (Exposed) Follow up for incidence of new cases Incidence measurement in both groups RR, Risk difference, Attributable risk, pop. A.R.

Cohort Study Present Future Disease Develop Exposed to Risk factor
Disease Not develop Comparison Disease Develop Not Exposed To Risk Disease Not develop Present Future 16

STEPS IN CONDUCTING COHORT STUDY
Select a cohort Baseline information Formation of study (exposed) and comparison (non-exposed) cohorts Follow-up Analysis for incidence and risk measurement in both the groups and comparison Study cohort or the exposure cohort is a group of persons exposed to the cause and in whom we are expecting the development of the disease in question e.g. smokers cohort.

FORMATION OF STUDY COHORT
Exclude From The Study Diseased persons! Exposed persons?? It is essential, before commencing the study, to exclude from among the study Group of exposed persons, Those persons already having the disease in question in any form, either in an incipient stage at cellular or biochemical level or in its early unrecognizable stages

SOURCES OF STUDY COHORT
1. General population 2. some selected groups like doctors, lawyers, etc 3. Special exposure groups like asbestos workers, miners, etc. Special population groups (doctors) or exposure groups (occupational groups) are taken for the study, as these are easily available and follow up will be easier. But they may reduce the external validity (generalization) to a certain extent, as these groups may not represent the general population.

INTERNAL COMPARISON Different subgroups of same heterogeneous cohort are compared For e.g. number of cases occurred in people smoking 2- packets /day can be compared with those developed in people smoking 4-packets/ day If the study cohort is heterogeneous in its exposure status and possessing different subgroups with different levels of exposure , these subgroups can be compared with each other regarding their extent of exposure and contributions (risks) to cause the disease. For example, number of cases occurred in people smoking 2- packets /day can be compared with those developed in people smoking 4-packets/ day.

EXTERNAL COMPARISONS When comparison is not possible or feasible to some reasons the rates are compared with the general population rates.

IDEAL COHORT WHILE SELECTING THE COHORT, A stable cohort,
A cooperative cohort, A committed cohort and A well-informed cohort IS TO BE USUALLY SELECTED.

Study and comparison cohorts can be chosen from special groups, exposure groups, hospital registers, medical records, and death certificates When registers and records are used for selecting a cohort e.g. historical cohort, complete and relevant records are to be used. Naturally, selection bias will creep in, as there is no randomization.

COHORT SELECTION TECHNIQUES
Select both cohorts from same population to which the results are to be extrapolated to enhance the generalizability or external validity. Select the exposed groups Measure the degree and duration of exposure Do follow up for incidence of outcomes. Both the study and comparison cohorts are to be selected from the same population to which the results are to be extrapolated to enhance the generalizability or external validity. In a cohort design, usually exposed groups are chosen and followed for the incidence of expected For exposure groups, degree and nature of exposure are to be considered.

COLLECTION OF BASE LINE INFORMATION
Collect in a similar fashion from the cohort regarding their exposure status Serves as a benchmark for future analysis Time, duration and extent of collection of data to be considered. To be collected in a similar fashion from both the groups regarding their exposure status and vulnerability to the disease or expected outcome This base-line information serves as a benchmark for future analysis and other considerations. Sources of information vary according to the availability and feasibility. They may be records and registers, medical examinations and investigations, interviews and environmental influence or contributions. Time, duration and extent of collection of data not only determine the design of cohort study but also influence the validity of the study. Collection through medical records and registers are to be carefully carried out considering the period of recording, quality and applicability. This is truer when information about investigations or tests is gathered for the study as their methodology and significance may change with the time. Study subjects may like the investigator or interested in the study and hence may give false or exaggerated information. Collection of information about dose-response relationship is vital and unless it is collected meticulously, study looses its validity.

FOLLOW UP Well-defined, standardized and Similar end points for proper follow up for both comparative cohorts Duration of follow up depends upon incubation periods (period when clinically appears) of the disease in question. Follow-up techniques and their duration of application should be similar to both the cohorts. Follow-up through the periodical medical examinations and mailed questionnaires is usually practiced. Good follow-up increases the internal validity of the study as it minimizes attrition. Cohort study is an incidence study. Hence the exposed and unexposed groups have to be followed for the occurrence of new cases or expected outcomes or end-points. . These end-points have to be standardized even before the follow-up is initiated. Any difference in endpoints will result in loss of validity The end points or outcomes expected are to be defined clearly for observation during follow up. These should be the same for both the cohorts otherwise analysis and inference will be false. Both the cohorts have to be followed up in similar fashion without any difference. The duration for follow up depends upon both the induction (initiation or onset of disease) and incubation periods (period when clinically appears) of the disease in question. It may be longer as in cancers, usually years. .

PROBLEMS FOR FOLLOW UP Time scarcity Paucity of trained personnel
The follow-up of large group (usual in cohort study) Resource crunch Time scarcity Paucity of trained personnel Attrition, loss on follow up is due to deaths, change in addresses, non-cooperation and withdrawal Diseases with high incidence, we choose for this study and the study needs large group of people. The follow-up of large group (usual in cohort study) over a longer period of time is difficult. Several problems, like resource crunch, time scarcity, paucity of trained personnel can occur or even the investigator may die. Hence, unless these requirements are adequately met, the cohort study should not be chosen. Moreover, the attrition, loss on follow up (due to deaths, change in addresses, non-cooperation and withdrawal) is another major draw back with this design. If this loss on follow-up is significant, the internal validity suffers. If this loss on follow-up is significant, the internal validity suffers

ATTRITION REDUCTION (loss to follow up)
By taking the informed consent may decrease the loss to follow up By enlisting the commitment to continue and cooperate in the study By tracing the lost ones and including them By considering the information of the lost persons at the time of analysis Try to keep the non-response at a low level to improve the validity To minimize loss, not only taking the informed consent but also enlisting the commitment to continue and cooperate in the study is to be given maximum consideration at the commencement of study itself. If lost during follow up, every effort must be made to trace and include them. The information of the lost persons either from registers (e.g. death registers) or from the neighbors and relatives is also to be considered at the time of analysis. It is always tried to keep the non-response at a low level to improve the validity.

ANALYSIS In a similar manner both for the study cohort and comparison cohort Due consideration to The loss on follow-up (attrition) The secondary information obtained from other sources (family members and neighbors) with regard to attrition RATES are calculated which are appropriate and feasible to a particular study. When calculated for specific time intervals, caution is observed with regard to time factor and particularly to the exact population involved under study. Time-person years will be a better denominator not only in those instances but also most of the times as there is an imminent danger of loss of the cohort subjects at any time. Cumulative incidence speaks better than crude incidence in studies of longer duration.

Design/Analysis of cohort study
Then follow to see the development of disease First select exposure Lung cancer NO Lung cancer Total Incidence rate Smokers a b a+b a/a+b Non-smokers c d c+d c/c+d If exposure is related to disease than a/a+b will be greater than c/c+d

Relative Risk Risk Ca. Lung Healthy Total Smokers (Risk Present) 325
175 500 Non-Smokers (Risk Absent) 85 415 Incidence of Ca. Lung among smokers = 325 ÷ 500 = 65 % Incidence of Ca. Lung in non - smokers = 85 ÷ 500 = 17 % Relative Risk = 65 ÷ 17 = 3.8 Smokers had 3.8 times higher risk of developing Lung Cancer as compared to non - smokers 31

INCIDENCE RATES Incidence among the exposed (new cases among the smokers) = (a/ a+b) Incidence among the non-exposed (new cases among the non-smokers) = (c/ c+d ) RISK Difference (RD) = (a /a+b) – (c/ c+d) RELATIVE RISK OR RISK RATIO (RR) = [a / (a+b)] / [c / c +d)] (incidence in the exposed (smokers) / incidence in the non-exposed (non-smokers) FOR SHORT PERIOD STUDIES:- Incidence rate (number of new cases / population at risk) Instantaneous incidence rate (new cases per unit time relative to disease-free population at risk at time t.) FOR LONGER STUDIES:- Cumulative incidence (disease risk during a specific interval/length of that interval) Cumulative incidence rate (summing the products of incidence rate and interval length over a series of intervals)

Strengths of cohort studies
Know that predictor variable [risk]was present before outcome [disease] variable occurred (evidence of causality) Directly measure incidence of a disease outcome Can study multiple outcomes of a single exposure (RR is measure of association)

Weaknesses of cohort studies
Expensive and inefficient for studying rare outcomes Often need long follow-up period or a very large population Loss to follow-up can affect validity of findings

Examples FRAMINGHAM HEART STUDY (1951)
ARTIFICIAL MENOPAUSE AND BREAST CANCER IN BOSTON AREA (1940) PROSPECTIVE STUDY ON SMOKING AND MORTALITY IN BRITISH DOCTORS (1951)

FRAMINGHAM HEART STUDY
1ST STEP - FORMATION OF COHORTS The study began in 1948 by recruiting an Original Cohort of 5,209 men and women between the ages of 30 and 62 from the town of Framingham, Massachusetts, who had not yet developed overt symptoms of cardiovascular disease or suffered a heart attack or stroke objectives To find out the risk factors for Coronary Artery Disease Study setting Framingham town was selected as it is compact, economically stable, more cooperative and represents U.S. population to a larger extent.

FRAMINGHAM HEART STUDY (1951)
II STEP - COLLECTING INFORMATION By initial examination and detailed interviews, the data was gathered. 82 Persons who had cardiovascular event at the initial examination were excluded out of 5209 subjects. The remaining 5127 subjects were classified according to the risk factors like high serum cholesterol, smoking, hypertension, body mass index and the presence of any other diseases. III STEP:- FOLLOW-UP Cohorts were followed for 20 years by means of repeated bi –annual examinations. Loss of cohort was more during the first 14 years (85%) due to incapacitation and migration. Indirectly, follow-up was carried through the secondary sources i.e. from the Framingham hospital records and town regional mortality registers. IV STEP:- ANALYSIS Average annual incidence= 0.0125 R.R. with high cholesterol=

III STEP:- FOLLOW-UP Cohorts were followed for 20 years by means of repeated bi –annual examinations. III STEP:- FOLLOW-UP Cohorts were followed for 20 years by means of repeated bi –annual examinations. Loss of cohort was more during the first 14 years (85%) due to incapacitation and migration. Indirectly, follow-up was carried through the secondary sources i.e. from the Framingham hospital records and town regional mortality registers. IV STEP:- ANALYSIS Average annual incidence= 0.0125 R.R. with high cholesterol=

IV STEP:- ANALYSIS Over the years, careful monitoring has led to the identification of the major CVD risk factors - high blood pressure, high blood cholesterol, smoking, obesity, diabetes, and physical inactivity - as well as a great deal of valuable information on the effects of related factors such as blood triglyceride and HDL cholesterol levels, age, gender, and psychosocial issues Since that time the Study has added an offspring cohort in 1971, the omni cohort in 1994, a third generation cohort in 2002, a new offspring spouse in 2003, and a second generation omni cohort in 2003.

ARTIFICIAL MENOPAUSE AND BREAST CANCER IN BOSTON AREA (1940)
1ST STEP - FORMATION OF COHORTS Women aged 55 years and younger and treated in two Boston hospitals between 1920 and 1940 were selected for the study. Details were collected from the pathological records and death certificates. STUDY COHORT includes those women with: - 1) Natural menopause (1479)2) Hysterectomy +bilateral oopherectomy (3241) CONTROL COHORT: -includes those women with :-1) Hysterectomy +unilateral 2) X-ray &irradiation group EXCLUDED: - 1) Prior mastectomy2) Prior breast cancer3) Treated for Pelvic malignancy4) Natural menopause <405) Premature removal of ovaries

2nd STEP STUDY COHORTs Hysterectomy with unilateral oophorectomy; Hysterectomy with bilateral oophorectomy; radium or X-ray treatment of the ovaries or uterus CONTROL COHORT: - lV. cholecystectomy

EXCLUDED: - 1) Prior mastectomy 2) Prior breast cancer 3) Treated for Pelvic malignancy 4) Natural menopause <40 years 5) Premature removal of ovaries 6) all who were not residents of Massachusetts

4 natural cohorts 1] Natural menopause—1479 women (including 953 women who underwent cholecystectomy and 526 women who were postmenopausal 2] Hysterectomy and bilateral oophorectomy—3241 women 3] Those undergoing hysterectomy and/or unilateral oophorectomy—2149 women 4] Radiation-induced artificial menopause—1518 women Example 1—A historical cohort study of the relation between artificial menopause and breast cancer Seven case–control studies performed between 1926 and 1962 all reported that artificial menopause (surgical removal of the uterus and/or ovaries) occurred significantly less frequently among breast cancer patients than among a variety of controls. Because the case–control studies did not present information about the extent of surgery (the effect of removal of only the uterus versus removal of the ovaries) or the effects of the age at which the artificial menopause occurred, it was decided to investigate these issues by means of a cohort study. The disadvantage of using a prospective cohort method in elucidating the relation between artificial menopause and breast cancer is that there is a long interval between the gynaecological procedure and the appearance of the disease in appreciable frequency. To reduce this delay it was decided to use the historical cohort approach. The cohorts were selected from the records of two technical hospitals in the Boston area. The study cohorts included all eligible patients seen at these hospitals from 1920 to Women aged 55 years or younger were eligible for inclusion in the study if they had undergone any of the following procedures as determined from surgical and pathological records: (i) hysterectomy; (ii) unilateral oophorectomy; (iii) bilateral oophorectomy; (iv) radium or X-ray treatment of the ovaries or uterus; (v) cholecystectomy. The last group served as a control cohort. Certain patients were excluded from the study: (i) women who had a prior mastectomy or a prior breast malignancy or who had undergone castration as part of the treatment for an existing breast tumour; (ii) women treated for pelvic malignancies; (iii) women who had previous removal of their ovaries or a history of natural menopause before the age of 40 years; (iv) women who did not survive their index admission; (v) all who were not residents of Massachusetts at the time of their index procedure. At the final editing of the study abstract forms and the elimination of duplicate records, there were 8387 patients in the study populations. They were subdivided into four ‘exposure’ categories. 1. Natural menopause—1479 women (including 953 women who underwent cholecystectomy and 526 women who were postmenopausal at the time of the gynaecological procedures for benign conditions). 2. Hysterectomy and bilateral oophorectomy—3241 women (this constitutes the surgically castrated group who were believed to have no residual ovarian activity). 3. Those undergoing hysterectomy and/or unilateral oophorectomy who, as far as could be ascertained from the surgical and pathological records, retained at least one intact ovary—2149 women (referred to as the ‘partial surgery’ group). 4. Radiation-induced artificial menopause—1518 women. The partial surgery group constituted a second control cohort and ‘sham operations’ with which to contrast the women subjected to hysterectomy and bilateral oophorectomy. It should be noted that it is not possible to relate the actual cohort studies to a clearly definable population. Although in this case adequate records were available for virtually every woman admitted to these hospitals who was eligible for the study, it is not known from what source population these women came. However, it is assumed that the reasons for coming to these particular hospitals were not correlated with both the type of procedure and the subsequent risk of developing breast cancer, i.e. they were not confounding factors (see section on biases below).

III STEP:- FOLLOW-UP For a range of years, cohorts were followed through various sources like health records, death certificates and tumor registry. Analysis Association of breast cancer was found with artificial menopause.

PROSPECTIVE STUDY ON SMOKING AND MORTALITY IN BRITISH DOCTORS (1951)
1ST STEP - FORMATION OF COHORTS A simple questionnaire was mailed out on 31 October 1951 to men and women on the Medical Register. The replies received from doctors ( from men and 6192 from women) were sufficiently complete to be used Example 2—A prospective cohort study of the relation between cigarette smoking and mortality: the British Doctors Study (Breslow and Day 1987, Appendix IA) By 1950 several case–control studies had been published and were in agreement in showing that a larger proportion of lung cancer patients had been heavy cigarette smokers and a smaller proportion had been non-smokers than patients with other diseases. Because of the possibility of a variety of biases in these case–control studies, a prospective study was launched in 1951 among the members of the medical profession in the United Kingdom. This group was chosen because it was felt that physicians would respond to mailed questionnaires, would report their smoking histories accurately, and could be followed economically through the death records of the Registrars-General and through the registries of the General Medical Council and the British Medical Association. It was felt that the relation of smoking to health among physicians would be similar to that in the general population. A simple questionnaire was mailed out on 31 October 1951 to men and women on the Medical Register. The replies received from doctors ( from men and 6192 from women) were sufficiently complete to be used. From a one-in-ten random sample of the register, it was estimated that this represented answers from 69 per cent of the men and 60 per cent of the women alive at the time of the inquiry. The degree of self-selection in those who replied was assessed in terms of the overall mortality using this one-in-ten sample. The standardized death rate of those who replied was only 63 per cent of the death rate for all doctors in the second year of the inquiry and 85 per cent in the third year. In the fourth to tenth years the proportion varied about an average of 93 per cent and there was no evidence of any regular change with the further passage of years. Evidently the effect of selection did not entirely wear off, but after the third year it had become slight. Doctors were chosen, as they will be ---- more interested in health matters, cooperate well, give accurate information about smoking histories easily accessible for follow-up.

PROSPECTIVE STUDY ON SMOKING AND MORTALITY IN BRITISH DOCTORS (1951)
COLLECTING INFORMATION Mailed questionnaires were used for obtaining information. Additional questionnaires were sent to know more about A) Smoking: Age When Smoking Started, Nature, Duration, Current Status, use of filter etc B) If Stopped, The Details Thus they were divided into 3 categories Current smokers Non smokers ex smokers III STEP:- FOLLOW-UP Further mailed questionnaires were sent twice to the women and thrice to the male doctors. Information about death was obtained from Registrar General of U.K, General medical register, British Medical association and from the underlying condition noted in the death certificates. IV STEP:- ANALYSIS Age specific rates (35-74 years)= 17.4% in non-smokers versus 24.9% in smokers RR =1.4% A.R=6.4%

III STEP:- FOLLOW-UP Information about death was obtained from Registrar General of U.K, General medical register, British Medical association and from the underlying condition noted in the death certificates.

IV STEP:- ANALYSIS Age specific death rates ( years) = 17.4% in non-smokers versus 24.9% in smokers

HURDLES IN COHORT STUDIES
Follow up Attrition Ethical problems LARGE SIZE IOF COHORTS WILL BE DIFFICULT TO FOLLOW WITHOUT LOSS. LOSS OF MORE THAN 10% WILL REDUCE THE VALIDITY, THOUGH ATTRITION IS UNAVOIDABLE . ETHICALLY AS IT IS NOT ADVISABLE AND HUMANE TO LEAVE THE THE PATIENTS TO GET THE DISEASE WITHOUT PREVENTION IS NOT CORRECT. HENCE,NECESSARY PERMISSIONS FROM ETHICAL COMMITTEE IS ESSENTIAL

large size of cohorts will be difficult to follow.
Loss of more than 10% will reduce the validity, though attrition is unavoidable Ethically as it is not advisable and humane to leave the patients to get the disease without prevention is not correct. Hence, necessary permissions from ethical committee is essential

SUMMARY Time consuming Expensive Less biased
Though cohort study is Time consuming Expensive Less biased It is better analytical method to prove causal hypothesis and direct risk measurement

Concurrent cohort study Retro-spective cohort study
Case-control study Concurrent cohort study Retro-spective cohort study Study group Comparison group Outcome measure

Case-control study Concurrent cohort study Retro-spective cohort study Study group Persons with disease (a+c) Comparison group Outcome measure

Case-control study Concurrent cohort study Retro-spective cohort study Study group Persons with disease (a+c) Exposed persons (a+b) Comparison group Outcome measure

Case-control study Concurrent cohort study Retro-spective cohort study Study group Persons with disease (a+c) Exposed persons (a+b) Comparison group Persons with out disease (b+d) Outcome measure

Case-control study Concurrent cohort study Retro-spective cohort study Study group Persons with disease (a+c) Exposed persons (a+b) Comparison group Persons with out disease (b+d) Non-Exposed persons (c+d) Outcome measure

Case-control study Concurrent cohort study Retro-spective cohort study Study group Persons with disease (a+c) Exposed persons (a+b) Comparison group Persons with out disease (b+d) Non-Exposed persons (c+d) Outcome measure Proportion of cases exposed a/a+c and controls exposed (b/b+d)

Case-control study Concurrent cohort study Retro-spective cohort study Study group Persons with disease (a+c) Exposed persons (a+b) Comparison group Persons with out disease (b+d) Non-Exposed persons (c+d) Outcome measure Proportion of cases exposed a/a+c and controls exposed (b/b+d) Incidence in exposed (a/a+b) and incidence in non-exposed (c/c+d) Incidence in exposed (a/a+b) and incidence in non-exposed (c/c+d)

Case-control study Concurrent study Retro-spective study
Measurement of risk Temporal relationship between exposure and outcome Multiple associations

Measurement of risk - Odds ratio Attributable risk Temporal relationship between exposure and outcome Multiple associations

Measurement of risk - Odds ratio Attributable risk Absolute risk Relative risk Temporal relationship between exposure and outcome Multiple associations

Measurement of risk - Odds ratio Attributable risk Absolute risk Relative risk Temporal relationship between exposure and outcome Hard to establish Multiple associations

Measurement of risk - Odds ratio Attributable risk Absolute risk Relative risk Temporal relationship between exposure and outcome Hard to establish Easily established Sometimes difficult to establish Multiple associations

Measurement of risk - Odds ratio Attributable risk Absolute risk Relative risk Temporal relationship between exposure and outcome Hard to establish Easily established Sometimes difficult to establish Multiple associations Establish association of disease with multiple exposures

Measurement of risk - Odds ratio Attributable risk Absolute risk Relative risk Temporal relationship between exposure and outcome Hard to establish Easily established Sometimes difficult to establish Multiple associations Establish association of disease with multiple exposures Establish association of exposure with multiple diseases

Time required for study Cost of study Best when Sample size Potential Bias

Time required for study Relatively short Generally long May be short Cost of study Best when Sample size Potential Bias

Time required for study Relatively short Generally long May be short Cost of study Usually inexpensive Expensive Less expensive Best when Sample size Potential Bias

Time required for study Relatively short Generally long May be short Cost of study Usually inexpensive Expensive Less expensive Best when Disease is rare Exposure is frequent Sample size Potential Bias

Time required for study Relatively short Generally long May be short Cost of study Usually inexpensive Expensive Less expensive Best when Disease is rare Exposure is frequent Exposure is rare Disease is frequent Sample size Potential Bias

Time required for study Relatively short Generally long May be short Cost of study Usually inexpensive Expensive Less expensive Best when Disease is rare Exposure is frequent Exposure is rare Disease is frequent Sample size small large Potential Bias

Time required for study Relatively short Generally long May be short Cost of study Usually inexpensive Expensive Less expensive Best when Disease is rare Exposure is frequent Exposure is rare Disease is frequent Sample size small large Potential Bias Assessment of exposure Assessment of outcome Both Assessment of exposure and Assessment of outcome

Home Work If a drug prevents mortality but does not affect cure, then which of the following will be true? Incidence will decrease Incidence will increase prevalence will decrease prevalence will increase prevalence of a disease: can only be determined by cohort study is the number of new cases in a defined population describes the balance between incidence, mortality and recovery is the best measure of disease frequency in etiological studies.

Prevalence is Rate Ratio Proportion Mode Study of time, place and person distribution of health related events is known as: Descriptive epidemiology Experimental epidemiology Analytical epidemiology Clinical epidemiology The changes that occur in a disease frequency over many years is called: Cyclic trend Secular trend Seasonal trend All of the above

State which type of mathematical parameter (ratio/proportion/rate), &
Which type of measure of disease frequency (cumulative incidence/incidence rate/prevalence) best describes each of the following? (Write your answer in the spaces allotted) Percentage of infants enrolled in a day-care center that contracted impetigo during the course of an epidemic? _________________ , __________________ Percentage of potential army recruits rejected because of poor vision. ___________________ , __________________ Number of colds experienced in a year per thousand people. ________________ , __________________

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