Having addressed the question about which measurement scale was used (“Statistics, not the dragon you think”), I want to talk about how many groups are being included in the evaluation and how those groups are determined.

The first part of that question is easy–there will be either one, two, or more than two groups.  Most of what Extension does results in one group, often an intact group.  An intact group is called a population and consists of all the participants in the program.  All program participants can be a very large number or a very small number.

The Tree School program is an example that has resulted in a very large number of participants (hundreds) .  It is a program that has been in existence for about 20 years.  Contacting all of these participants would be inefficient.  On the other hand, the 4H science teacher training program involved a  small number participants (about 75) and has been in existence for 5 years. Contacting all participants would be efficient.

With a large population, choosing a part of the bigger group is the best approach.  The part chosen is called a sample and is only a part of a population.  Identifying a part of the population starts with the contact list of participants.  The contact list is called the sampling frame.  It is the basis for determining the sample.

Identifying who will be included in the evaluation is called a sampling plan or a sampling approach.  There are two types of sampling approaches–probability sampling and nonprobability sampling.  Probability sampling methods are those which assure that the sample represents the population from which it is drawn.  Nonprobability sampling methods are those which are based on characteristics of the population.  Including all participants works well for a population with less than 100 participants.  If there are over 100 participants, choosing a subset of the sampling frame will be more efficient and effective.  There are several ways to select a sample and reduce the population to a manageable number of participants.  Probability sampling approaches include:

  • simple random sampling
  • stratified random sampling
  • systematic sampling
  • cluster sampling

Nonprobability sampling approaches include:

  • convenience sampling
  • snowball sampling
  • quota sampling
  • focus groups

More on these sampling approaches later.

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