BMTRY 738: The Study Population

Population/Sample Population: a complete set of people with a specified set of characteristics Geographic Demographic Clinical Temporal Sample: subset of a population

Population/Sample Target Population Accessible/Source Population External Validity (Generalizability) Accessible/Source Population External Validity Intended Population Internal Validity Actual Subjects

Target Population Step 1: Defining Target Population Better not to have too many inclusion or exclusion criteria Increases eligible participants Increases generalizability Exclusion High likelihood of being lost to follow-up Unable to provide good data High risk of a side effect

Accessible/Source Population Step 2: Defining Accessible/Source Population Representative of the Target population Accessible Patients at a given clinic Children at a single school Trade off between feasibility and bias Aims of the study are important Descriptive study, must be representative Incidence Prevalence Associations between exposure and disease

Accessible/Source Population Step 2: Defining Accessible/Source Population Clinical versus community populations Population-based study More expensive Takes longer Clinical study Easy to access patients with a disease Unable to get prevalence or incidence in the community Do the clinics in the study have patients representative of patients in the general population

Intended Population Step 3: Defining Intended Population Sample of the source population Convenience sampling Anyone who meets the eligibility criteria Quick and cheap Consecutive sample, ask everyone within a certain time frame Volunteer bias Probability sampling Simple random sample Stratified random sample (want to ensure you have enough minority participants in your study) Systematic samples Cluster sampling

Actual Population Step 4: Actual Population Response Rate: actual (n)/ intended (n)

Project Part 1 If you are not using word, don’t just write on the sheet. Use an extra sheet of paper. Try to write complete sentences, or at least not one word answers.

Project Part 1 Define the target population for the BRFSS. Include inclusion and exclusion criteria. The target population is adult US citizens in 53 states and territories. Overview

Project Part 1 Define the accessible or source population for the BRFSS. Include inclusion and exclusion criteria that pertain to differentiating between the source and target population. Why do you think this source population was chosen? What are its strengths and weaknesses? Source Population: adults in 53 states/territories who have a landline home telephone and answer unknown numbers since this is a RDD telephone survey. Why: This population was chosen for feasibility in both time and money to access a large sample population across a large distance with varying interview times. Strengths Weaknesses

Project Part 1 List groups of people who are systematically excluded from the BRFSS source population? Non-landline home telephone users; institutionalized individuals: nursing homes, jails, prisons, rehab/long-term care hospitals; university/college students in university housing (dorms, university apartments);

Project Part 1 What is a sampling frame? What sampling frame is used for the BRFSS? A sampling frame is a listing of units (people) from which the sample is drawn. The sampling frame used for the BRFSS is a list of all active telephone numbers in the US that could possibly be assigned to households. Other sampling frames?

Project Part 1 Define the intended sample for the BRFSS. The intended sample for the BRFSS is a representative sample of US citizens in terms of geographic distribution based on the US Census. Those numbers in highly populated areas are called at a higher rate.

Project Part 1 Response Rate BRFSS 2007 South Carolina Pennsylvania Actual People (n)/ Intended (n)

Project Part 1 Data Weighting Representative sample Want your results to reflect the true population not who responded to the questionnaire. Design – oversample Conducting – response rate may differ BRFSS – nonresponse; noncoverage; adjusts variables of age, race, and sex between the sample and the entire population

Project Part 1 Population Based Sampling frame Is the source population a representative sample of the population?

Project Part 1 Population Based versus Clinical Generalizable? Internal or external validity?

Project Part 1 If you invite members from the community to participate in a cohort study, is this population-based research?

Project Part 1 BRFSS Study Design?