What Is a sample size?

Written by jason brick
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What Is a sample size?
The sample subjects can be plants, animals or products as well as people. (Image Source/Stockbyte/Getty Images)

Sample size, simply defined, is the number of participants in a given study. It's an important aspect of the statistics that go into analysing the results of a research project, because it is integral to calculating all of the results of that study. Some studies consist of a single sample, while others will compare the results of several samples. In the latter, sample size might refer to any subgroup or to participants in the study as a whole.

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Validity

Sample size is an important part of establishing the validity of a study. In general, the larger the sample size, the more likely the research is to be accurate. A poll of eight people is less likely to be representative of a research group as a whole than asking the same questions of 8,000 people. Depending on the research being done, sample size for validity may be rated in number of people participating or the percentage of eligible people participating. A study of television-watching habits would be rated on the raw number of participants. A survey of graduating seniors might be rated based on the percentage of seniors interviewed.

Deviation

Sample size also relates strongly to deviation, the margin by which the results are likely to be wrong. Many studies will report results as a number, plus or minus a specifically calculated deviation. Smaller size samples have wider deviations than larger size samples because outliers will have a greater significance.

Split samples

Much scientific research splits the whole sample group into at least two subgroups: the subject group and the control group. The subject group is subjected to whatever the research is studying. The control group is either left alone or made to think they are being worked on, but really given a placebo. This allows researchers to know what happens both in the presence or the absence of a variable. Some research further subdivides groups, subjecting each to a different protocol.

Averages

The sample size is a vital part of determining the average result of a study. There are three kinds of averages: the median, the mode and the mean. The median average is the result at the centre of a range of results. For example, the median of five results 2, 4, 6, 12 and 108 is 6. The mean average is what most people think of when they say "average": the total of all results, divided by the sample size. For the above five results, the mean average is 26.4. The mode average is the result that occurred most often. For the five results 2, 4, 4, 6 and 12, the mode average is 4.

Other factors

Other statistics relating to a sample are significant in analysing the results of a study. Two of the most common are breadth and longitude. Breadth refers to how wide a sample of people is contained within the sample. For example, a study of everybody in a single town of 10,000 would not be nearly as broad as a nationwide survey of 5,000 people. Longitude refers to how long a particular sample is studied. A "man-on-the-street" survey has very low longitude, while some medical studies monitor participants for decades.

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