I am doing a research on two groups, one experimental and one control, which are not randomly selected. In fact the two groups are *intact groups*. The purpose is to find the effect of the number of languages that the learners know on their academic achievement. The selected design is Pretest Posttest Nonequivalent Group.

Which statistical method should I use to analyze the data?

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#### Best Answer

It sounds like you want some form of regression, depending on how your dependent variable (academic achievement) is measured. If it's something like grade point average, then linear regression is a good start.

Then the question is how to measure "languages known". Clearly, it's a count variable (that is, it has to be a non-negative integer; in this case, a positive integer) but (depending on where you are doing this) you may have a huge preponderance of 1's.

You will also want to include covariates that relate to achievement. You *might* want to do some sort of matching, possibly propensity scores.

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