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My understanding of sample size is that it is a portion of the population that serves to represent the population.

However, I'm not sure if the sample size of an investigation to investigate how X variable affects Y variable (in which a range of X is investigated to see how it affects Y) refers to the number of repeats conducted at each X or the total number/range of X used.

For example, for an investigation with the context attached below, would the 3 repeats conducted at each brain mass (e.g. Cavy A, Cavy B and Cavy C) be the sample size? Or would the range of brain mass i.e. 5 (Cavy, Gerbil, Hamster, Mouse and Rat) be the sample size?

I've been told by my teacher that the 3 repeats conducted at each brain mass is the sample size for this context, but I'm not sure if that's correct. Thanks.

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2 Answers 2

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The sample is a collection of observations of a population that serves to represent the population. The sample size is the number of such observations. Part of what's throwing you off here, I think, is that there are different sample sizes depending on what you are talking about.

Usually, there is one number, represented by N, that reflects the total sample size. In your case, the total number of observations is N = 15. If you were indicating the sample size in each group, you could say n = 5. Note the usage of a lower-case n.

In some cases, when observations are "nested" into different groups or populations, you may also want to indicate both the sample size at the level of the group/population and the sample size at the level of individuals nested in those populations. Sample size is typically reported at both levels when you are doing multilevel modeling, for example. In that case, the group-level sample size would be 3, and the individual-level sample size would be 15.

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The sample size the number of for example :people you have included in your study. You can do as many repeated measures as you like. So for example you survey 350 people asking them ten different questions. 350 is your sample size. There are an endless number of ways to calculate how big your sample size should be to be representative of the population you are studying to be able to measure significance. It very much depends on the kind of data and statistical tests you are doing. In the example you gave, they used 3 animals of each sort, 5 different animals. Your sample size is n=15. Or if you break it down by animal type n=3 per group

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