Chi square test

Sucheta Tripathy, Biostatistics course work IICB,Nov, 2013

Definitions

TRUE

FALSE

•Model or Hypothesis•Null Hypothesis• There is no significant difference between the 2.

•Goodness of fit

What you need• A probability value• Degree of freedom• A contingent tableDetermine if the deviation is due to chance

Accept

Reject

10%

Chi Square Test

Example 1

• Mendellian law of dominance

Aa X Aa

A a X A a

A -> Tall (Dominant)a -> Dwarf (recessive)Aa is …….

AA Aa Aa aa

639 Tall and 281 dwarf

Chi square requires that you have numeric values

Chi square should not be calculated if the expected value is less than 5

Example

9:3:3:1

ExampleNumber of sixes Number of Rolls 0 48 1 35 2 15 3 03

p1 = P(roll 0 sixes) = P(X=0) = 0.58 p2 = P(roll 1 six) = P(X=1) = 0.345 p3 = P(roll 2 sixes) = P(X=2) = 0.07 p4 = P(roll 3 sixes) = P(X=3) = 0.005

P(k out of n) = n!

pk(1-p)(n-k)

k!(n-k)!

http://www.mathsisfun.com/data/binomial-distribution.html