Hypothesis testing - ttest difference in 1-tail versus 2-tail


I am runnings some Ttests, and when I conduct a one-tailed test to see if Variable "a" is greater than Variable "b" the result says to reject the null hypothesis, but when I conduct a two-tailed test to see if Variable "a" is different than variable "b" the result says not to reject the null hypothesis.

Which do I use????



TS Contributor
That depends on the purpose of your experiment. Are you interested in a specific direction of change? Then use the one-tailed test as it is more sensitive. If you want to detect any change regardless of direction, use the 2-tailed test.

However, you should decide in advance, not perform a 2-tail test then follow up with a 1-tail test. This increases your chance of making a Type 1 error.