1 Sampling to Study Drug Use

2 Sampling to Study Drug Use: Objectives Describe the principles on which sampling is basedDescribe the principles on which sampling is based Identify and describe different sampling methods Identify and describe different sampling methods Select a sample of health facilities from a list using at least two different methods Select a sample of health facilities from a list using at least two different methods Compare the results of the methods Compare the results of the methods Instruct others how to sample for drug use studies Instruct others how to sample for drug use studies

Sampling to Study Drug Use3 What Is Sampling? Sampling is a process by which we study a small part of a population to make judgments about the entire population.

Sampling to Study Drug Use4 Sampling involves selecting a number of units from a defined population.

Sampling to Study Drug Use5 Sampling Definitions Sampling Unit - The thing that is sampled: for example, a person, clinical episode, or health facilitySampling Unit - The thing that is sampled: for example, a person, clinical episode, or health facility Study Population - All the sampling units that could possibly be included in the sampleStudy Population - All the sampling units that could possibly be included in the sample Sampling Frame - A list of all the available sampling units in the study populationSampling Frame - A list of all the available sampling units in the study population

Sampling to Study Drug Use6 A Representative Sample A representative sample has all the important characteristics of the study population from which it is drawn.

Sampling to Study Drug Use7 Sampling Methods Two categories of sampling methods: - Nonprobability sampling - Probability samplingTwo categories of sampling methods: - Nonprobability sampling - Probability sampling

Sampling to Study Drug Use8 Nonprobability Sampling Methods Convenience Sampling - Study units available at the time of data collection are selected for the sampleConvenience Sampling - Study units available at the time of data collection are selected for the sample Quota Sampling - Different categories of sample units are included until a certain number has been reached in each categoryQuota Sampling - Different categories of sample units are included until a certain number has been reached in each category

Sampling to Study Drug Use9 Probability Sampling Methods Simple Random SamplingSimple Random Sampling Systematic SamplingSystematic Sampling Stratified SamplingStratified Sampling Cluster SamplingCluster Sampling Multistage SamplingMultistage Sampling

Sampling to Study Drug Use10 Simple Random Sampling Used in situations where the number of sampling units is relatively smallUsed in situations where the number of sampling units is relatively small Process: - Identify all possible units available for sampling - Decide on the size of the sample - Choose units by a lottery methodProcess: - Identify all possible units available for sampling - Decide on the size of the sample - Choose units by a lottery method

Sampling to Study Drug Use11 Systematic Sampling with Equal Probability Numbered list of all possible unitsNumbered list of all possible units # units  desired sample size = sampling interval# units  desired sample size = sampling interval –For example, to select 20 health centers from a list of 46, the sampling interval is 46/20 = 2.3 Random # x sampling interval = random startRandom # x sampling interval = random start –For example, if the random number is 0.183, calculate × 2.3 = 0.421, which rounds upward to 1 Round number up to choose sample unitRound number up to choose sample unit Add sampling interval to random start for subsequent unitsAdd sampling interval to random start for subsequent units –For example, = or Facility = or Facility = or Facility 8 and so forth

Sampling to Study Drug Use12 Systematic Sampling with Probability Proportional To Size List where the units are sorted in decreasing order by some measure of size (like population or number of visits)List where the units are sorted in decreasing order by some measure of size (like population or number of visits) Calculate the cumulative totalCalculate the cumulative total Cumulative total  sample size = sampling intervalCumulative total  sample size = sampling interval Random # x sampling interval = random startRandom # x sampling interval = random start Choose first unit with cumulative total resultChoose first unit with cumulative total result Add sampling interval to previous total for subsequent units.Add sampling interval to previous total for subsequent units.

Sampling to Study Drug Use13 Stratified Sampling Used when the sampling frame contains clearly different categories (strata)Used when the sampling frame contains clearly different categories (strata) –For example, Urban and rural facilitiesUrban and rural facilities Facilities with and without doctorsFacilities with and without doctors Government and mission facilitiesGovernment and mission facilities Process: - Organize the list of sampling units by stratum - Select units within each stratum using a random method (simple random sampling or systematic sampling)Process: - Organize the list of sampling units by stratum - Select units within each stratum using a random method (simple random sampling or systematic sampling)

Sampling to Study Drug Use14 Cluster Sampling Used when, for logistic reasons, it is easier to select sample units in groupsUsed when, for logistic reasons, it is easier to select sample units in groups Process - Select a cluster of sample unitsProcess - Select a cluster of sample units –Example: health center with multiple prescribers Include the entire cluster or select a subsample or Select a random sample unit to start each cluster (a house, a patient, etc.) - Include neighboring sample units until a certain cluster size is reachedInclude the entire cluster or select a subsample or Select a random sample unit to start each cluster (a house, a patient, etc.) - Include neighboring sample units until a certain cluster size is reached

Sampling to Study Drug Use15 Multistage Sampling Randomly select primary sampling units at the first stage:Randomly select primary sampling units at the first stage: –Specific communities –Specific health facilities Within the primary sampling units, randomly select the final sampling units at the second stage:Within the primary sampling units, randomly select the final sampling units at the second stage: –Drug use encounters –Patients –Households Sometimes in complex samples, additional stages are neededSometimes in complex samples, additional stages are needed

Sampling to Study Drug Use16 Sample Size The optimal sample size is often a compromise between what is statistically Desirable and what is practically Feasible.The optimal sample size is often a compromise between what is statistically Desirable and what is practically Feasible. In general, a minimum sample size is 30.

Sampling to Study Drug Use17 Sample Size Sample size Sample size Range of Confidence % % % % %Range of Confidence % % % % % Confidence Intervals If the real figure is 50%

Sampling to Study Drug Use18 Conclusion Principles of sampling should be understood by everyone involved in drug use studiesPrinciples of sampling should be understood by everyone involved in drug use studies Enumerators and supervisors should understand that any facility, any prescription, or any patient should have a chance to be includedEnumerators and supervisors should understand that any facility, any prescription, or any patient should have a chance to be included Make every effort to avoid bias in selecting units for studyMake every effort to avoid bias in selecting units for study

Sampling to Study Drug Use19 Activity 1 Sample Selection In this exercise, groups will use the different sampling methods on a real data set to select a sample. The results of the different sampling methods will be compared.