Course work IICB, 2013; Biostatistics

1. Chi square test
Sucheta Tripathy, Biostatistics course
work IICB,Nov, 2013

2. Definitions
•Model or Hypothesis
•Null Hypothesis
•There is no significant difference between the 2.
TRUE
FALSE
•Goodness of fit

3. What you need
• A probability value
• Degree of freedom
• A contingent table
Determine if the deviation is due to chance
Accept
10%
Reject

4. Chi Square Test

5. Example 1
• Mendellian law of dominance
A -> Tall (Dominant)
a -> Dwarf (recessive)
Aa is …….
Aa X Aa
A
a X A
a
639 Tall and
281 dwarf
AA Aa Aa aa
Chi square requires that you have numeric
values
Chi square should not be calculated if the
expected value is less than 5

6. Example
9:3:3:1

7. Example
Number of sixes
0
1
2
3
Number of Rolls
48
35
15
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!
k!(n-k)!
http://www.mathsisfun.com/data/binomial-distribution.html
pk(1-p)(n-k)