Chi Square

This week in AP biology we learned a very important formula called chi square. Chi square is relating to or denoting a statistical method assessing the goodness of fit between observed values and those expected. Chi square as an odd looking formula, (which I will put a picture of at the end of the blog) the odd looking E stands for summation or the sum, the O means what was observed and the regular E means what you expected. If there are multiple observations and expectations, you add each one. For example you have 1600 cards with four suits. You would expect 400 cards of each suit, but when you count the cards, the number is completely random. So you would have to subtract the observed from the expected, then square that number, than divide that number by the expected number, and you will have to continue doing that for each suit of cards and add up each answer to get the final answer. Once you get the final answer, you have to look at your degrees of freedom chart and in our AP Biology class, we look at the .05 chart which means that we are 95% sure. A very important thing to know about the degrees of freedom is that you see how many observation there are and subtract it by one. So if there are 4 observations, you would look at the third degree of freedom. Also if the final answer you got from doing chi square is lower than the degrees of freedom, it accepts the null hypothesis and if its higher, it declines it. The null hypothesis is when there is no significant  difference between what is observed and what is observed. Chi square is pretty simple and easy to do once you get the hang of it.