How do we calculate Chi-Square from this given information.

Question

You are interested in the number of carnivorous plants in an area of campus that might be impacted by construction. We counted the number of each type of plant and have found 10 sundews, 15 butterworts, 20 hooded pitcher, 22 bladderworts, 12 purple pitchers and 25 hybrid pitchers. How do you calculate Chi-Square from this information?

Answer 1

You can't... what you have listed is the OBSERVATION; that's half the battle. chi-square is a measure of how closely the observed data match those that were expected. So you need a hypothesis that generates predictions about the expected numbers. These then become the Obs and Exp values in the chi-square equation.

 

Example--if you had anticipated EQUAL numbers of all the different sorts, you have (10+15+20+22+12+25)=94 total plants. Six kinds, so if equally distributed, you would expect 94/6= 15.67 of each. 

 

You then take the difference between each count vs. the expected count, square the value, divide by expectation, sum all... and you're off to the races! (or to a p-value table :-)