Notes

- Enter Category ID, Observed Count values and (optional) Historical Counts. Do not modify any other part of this worksheet.
- If optional historical counts are not specified, chi-square is calculated using equal expected proportions.
- If optional Historical Counts are specified, a value must be entered for each observed count.
- The Chi-Square statistic requires that no more than 20% of cells have an expected count less than 5 (and none of the cells have an expected count less than 1). If this assumption is not satisfied, the Chi-Square approximation may be invalid and Exact or Monte Carlo P-Values should be used. In this example 40% of the cells have an expected count less than 5 so Exact should be used.
- This example shows that the Exact P-Value is 0.5013 and the “large sample” Chi-Square P-Value is 0.4897.
- Chi-Square Exact solves the permutation problem using enhanced
enumeration. For further details refer to the Appendix
**Exact and Monte Carlo P-Values for Nonparametric and Contingency Test.** - It is important to note that while exact P-Values are “correct,” they do not increase (or decrease) the power of a small sample test, so they are not a solution to the problem of failure to reject the null due to inadequate sample size.
- For data that requires more computation time than specified, Monte Carlo P-Values provide an approximate (but unbiased) P-Value that typically matches exact to two decimal places using 10,000 replications. One million replications give a P-Value that is typically accurate to three decimal places. A confidence interval (99% default) is given for the Monte Carlo P-Values.

**Monte-Carlo Example**

- The Exact P-Value for this example is solved very
quickly, so Monte Carlo is not needed, but we will run it for continuity in
the example. Click
**Monte Carlo P-Value**. Select**Number of Replications**= 10000 and**Confidence Level for P-Value**= 99%.

**Tip**: The Monte Carlo 99% confidence interval for P-Value is not the
same as a confidence interval on the test statistic due to data
sampling error. The confidence level for the hypothesis test
statistic is still 95%, so **all reported P-Values less than .05 will
be highlighted in red** to indicate significance. The 99% Monte Carlo
P-Value confidence interval is due to the uncertainty in Monte Carlo
sampling, and it becomes smaller as the number of replications
increases (irrespective of the data sample size). The Exact P-Value
will lie within the stated Monte Carlo confidence interval 99% of
the time.

- Click
**OK**. Results

The Monte Carlo P-Value here is 0.5012 with a 99% confidence interval of 0.4896 to 0.5128. This will be slightly different every time it is run (the Monte Carlo seed value is derived from the system clock). The true Exact P-Value = 0.5013 lies within this confidence interval.

**Small Sample Exact Sample**

- Now we will consider a small sample problem. Enter the following values for sample data in the yellow highlight region. Note that the displayed Monte Carlo (or Exact) P-Values are cleared when new data is entered in the template:

This example is adapted from Mehta, C.R. and Patel, N. R., IBM
SPSS Exact Tests, IBM Corp., page 44.

- Click
**Exact P-Value**. Select**Time Limit for Exact Computation**= 60 seconds.

- Click
**OK**. Results:

Note that the Exact P-Value is 0.0523 which is a “fail-to-reject”
of the null hypothesis (H0), but the “large sample” or “asymptotic”
Chi-Square P-Value incorrectly rejected H0 with a P-Value of 0.046.
The exact P-Value matches that given in the reference. The error can
also go the other way, where a large sample Chi-Square P-Value is a
“fail-to-reject” of the null hypothesis and the Exact P-Value is a
rejection of H0.

In conclusion, always use the Exact (or Monte Carlo) P-Value when the Chi-Square large sample assumptions are not met.

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