POPULATION SAMPLING Nigel Paneth

SCIENTIFIC CONCERNS THAT UNDERLY SAMPLING INTERNAL VALIDITY. The extent to which we can be sure that the result we find within our study sample is real and valid. EXTERNAL VALIDITY. The extent to which we can be sure that the results we find in our study sample can be generalized to other samples.

SAMPLING SCHEMES TO AVOID HAPHAZARD SAMPLE - anyone who comes in, or mixtures of sources. VOLUNTEER SAMPLE - all who wish to participate.  CONVENIENCE SAMPLE - the easiest sample for an investigator to obtain. BIASED SAMPLE - A sample constituted in such a way that inferences will be misleading.

Sampling schemes to use SIMPLE RANDOM SAMPLE - Every individual in the sampling unit has the same chance of being chosen. STRATIFIED RANDOM SAMPLE - The sampling unit is divided according to some characteristic, and all individuals with the same characteristic have the same chance of being chosen, which is different for each characteristic.

SIMPLE AND STRATIFIED RANDOM SAMPLES In a simple random sample, you may sample one out of five men and one out of five women. Both men and women have the same chance of being sampled. In a stratified random sample, you may take one out of five women and one out of ten men. Women are twice as likely to be sampled.

MORE SAMPLING SCHEMES TO USE 3. SYSTEMATIC SAMPLE - A simple random sample, but instead of randomly selecting each individual, the first is selected, and then all others are at a systematic distance from the first (e.g. every tenth case thereafter). Potentially biased, use with caution. 4. CLUSTER SAMPLE - Same as simple random sample, but the unit of interest to be sampled is ecological, not individual. 5. TWO-STAGE SAMPLE - Sampling is done in two consecutive stages. Usually a cluster sample (e.g. houses) is followed by simple random sample (individuals within houses).

WHAT’S WRONG HERE? “A second, smaller study involved women recruited ….. with the cooperation of DES Action, an organized consumer action group. Participants in this group responded to a letter sent to the DES Action mailing list, to posters, or to presentations in classes on the Berkeley campus. Each participant completed a questionnaire and donated a sample of blood. Based on responses to questions regarding the use of medications such as DES by their mothers during their pregnancies, the participants were grouped into those exposed to DES in utero, those not exposed, and those unsure of their status with regard to DES exposure. ……The two groups were further limited to include only white females aged 25-35 years, which yielded a group of 33 DES-exposed individuals and a group of 21 unexposed controls.”