These next steps will tell you how to calculate the p-value from t-test or its critical values, and then which decision to make about the null hypothesis. So, you've decided which t-test to perform. The change in blood pressure in patients before and after administering some drug.The change in student test performance before and after taking a course.In particular, you can use this test to check whether, on average, the treatment has had any effect on the population. This test is sometimes referred to as an independent samples t-test, or an unpaired samples t-test.Ī paired t-test is used to investigate the change in the mean of a population before and after some experimental intervention, based on a paired sample, i.e., when each subject has been measured twice: before and after treatment. The average difference in the results of a math test from students at two different universities.The average difference in weight gain in two groups of people: one group was on a high-carb diet and the other on a high-fat diet.In particular, you can use this test to check whether the two groups are different from one another.
The average weight of people from a specific city - is it different from the national average?Ĭhoose the two-sample t-test to check if the difference between the means of two populations is equal to some pre-determined value, when the two samples have been chosen independently of each other.The average volume of a drink sold in 0.33 l cans - is it really equal to 330 ml?.Note: Microsoft Excel recommends using the new VAR.S function.Your choice of t-test depends on whether you are studying one group or two groups:Ĭhoose the one-sample t-test to check if the mean of a population is equal to some pre-set hypothesized value.
VAR and VAR.S produce the exact same result. Take the square root of this result to find the standard deviation based on a sample.ģ. Note: you already knew this answer (see step 1 under STDEV.S). The VAR.S function below estimates the variance based on a sample. Take the square root of this result to find the standard deviation based on the entire population.Ģ. Note: you already knew this answer (see step 5 under STDEV.P). The VAR.P function below calculates the variance based on the entire population. Sometimes it's easier to use the variance when solving statistical problems.ġ. Variance is the square of the standard deviation. Note: why do we divide by n - 1 instead of by n when we estimate the standard deviation based on a sample? Bessel's correction states that dividing by n-1 instead of by n gives a better estimation of the standard deviation. Fortunately, the STDEV.S function in Excel can execute all these steps for you.
Repeat steps 1-5 above but at step 5 divide by n-1 instead of N.ģ. The STDEV.S function uses the following formula: You only have the test scores of 5 students. For example, you're teaching a large group of students. The STDEV.S function (the S stands for Sample) in Excel estimates the standard deviation based on a sample. Fortunately, the STDEV.P function in Excel can execute all these steps for you. Divide by the number of data points (N = 5).ħ. For each number, calculate the distance to the mean.ģ. The STDEV.P function uses the following formula: The entire population consists of 5 data points. You have the test scores of all students. For example, you're teaching a group of 5 students. The STDEV.P function (the P stands for Population) in Excel calculates the standard deviation based on the entire population. As a result, the numbers have a high standard deviation. The numbers below also have a mean (average) of 10.Įxplanation: the numbers are spread out. As a result, the numbers have a low standard deviation.ģ. The numbers below also have a mean (average) of 10.Įxplanation: the numbers are close to the mean.
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