MathBench > Statistical Tests

Chi-square Tests

Detour stop #2: what's a Lookup Table?

So, so far we have a chi-square-calc, which has a p-value associated with it. This would be fine and dandy IF we actually knew what that p-value was. But we don't. And in fact, finding out the p-value for any given chi-square-calc would involve a complicated mathematical formula. Believe it or not, biologists are not actually big on complicated mathematical formulas (or formuli either). So instead we have a lookup table. Or as I like to say, a Magic Lookup Table, because for our purposes, it might as well have appeared magically.

What the lookup table tells you is, for your specific dataset, what the chi-square calc is that would correspond with p=0.05. This special number is called the "chi-square-crit", as in the critical value or threshold value of the chi-square-calc.

chi-square lookup table


And how do you know that this chi-square-crit is the one and only chi-square-crit that fits your exact dataset? It turns out that you only need to know one thing about your dataset, which is how many rows are in the chi-square table. If your chi-square table has 2 rows (like ours), then you look up the chi-square crit under df = 1 (cuz 2-1 = 1, more about that on the next page).