Why Do Many Of Today's Research Place Heavy Emphasis On Correctly Defining A Target Population

Target Population

1. Why do many of today’s research situations place heavy emphasis on correctly defining a target population rather than a total population?

The target population consists of the complete group of elements (people or objects) that are identified for investigation based on the objectives of the research project. A precise definition of the target population is essential because it will represent the total population and since it is too costly and time consuming to take a survey of every single person or object than researchers must make sure that their target population is precise.

Sample Size 2. What factors must be considered in determining sample size?

Consider these things when selecting a sample size When choosing

As we will see below, when choosing sample sizes we need to select risk values. If the decisions we will make from the sampling activity are very valuable, then we will want low risk values and hence larger sample sizes.

Prior information If our process has been studied before, we can use that prior information to reduce sample sizes. This can be done by using prior mean and variance estimates and by stratifying the population to reduce variation within groups.

Inherent variability We take samples to form estimates of some characteristic of the population of interest. The variance of that estimate is proportional to the inherent variability of the population divided by the sample size:

Var(p̂ )≈σ2n with p̂ denoting the parameter we are trying to estimate. This means that if the variability of the population is large, then we must take many samples. Conversely, a small population variance means we don't have to take as many

There must be a statement about what is expected of the sample. We must determine what is it we are trying to estimate, how precise we want the estimate to be, and what are we going to do with the estimate once we have it. This should easily be derived from the goals.

2. We must find some equation that connects the desired precision of the estimate with the sample size. This is a probability statement. A couple are given below; see your statistician if these are not appropriate for your situation.

3. This equation may contain unknown properties of the population such as the mean or variance. This is where prior information can help.

4. If you are stratifying the population in order to reduce variation, sample size determination must be performed for each stratum.

5. The final sample size should be scrutinized for practicality. If it is unacceptable, the only way to reduce it is to accept less precision in the sample estimate.

Sampling 3. Summarize why a current telephone directory is not a good source from which to develop a sampling frame for most research