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help for chitesti
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Chi-square test for univariate frequency distributions

chitesti #obs1 #obs2 [...] [ \ #exp1 #exp2 [...] ] [ , nfit(#)
replace list_options ]

Description

The input for chitesti must consist of

either a single row of values

or two rows of values separated by a backslash \.

The first row is taken to be observed frequencies, which must be zeros or
positive integers. The second row, if present, is taken to be expected
frequencies under some hypothesis, which must be positive.  These may be
given either as numbers or as numeric expressions without embedded
spaces. If the second row is not present, the expected frequencies are
taken to be equal, i.e. equal to the mean of the observed frequencies.

The display includes the Pearson chi-square statistic and its P-value for
a test of the hypothesis, the likelihood-ratio chi-square statistic and
its P-value, observed frequencies, expected frequencies, residuals
(observed - expected), and Pearson residuals, defined as (observed -
expected) / sqrt(expected).

Any cells with expected frequencies less than 5 are flagged.

Options

nfit() indicates the number of parameters that have been estimated from
the data. This number will be subtracted from (number of cells - 1)
to give the number of degrees of freedom. The default is 0.

replace indicates that the observed and expected frequencies are to be
left as the current data in place of whatever data were there. These
variables will be called observed and expected.

list_options are options of list.

Examples

Breiman (1973, p.191) lists the frequencies of the digits 0 ... 9 in the
first 608 decimal places of pi as 60 62 67 68 64 56 62 44 58 67.

. chitesti 60 62 67 68 64 56 62 44 58 67

Breiman (1973, p.191) also gives data from one of Mendel's experiments.
He observed 315 round yellow peas, 108 round green peas, 101 wrinkled
yellow peas and 32 wrinkled green peas.  According to theory, the
expected frequencies should be in the ratio 9:3:3:1.

. chitesti 315 108 101 32 \ 556*9/16 556*3/16 556*3/16 556*1/16

. gen lastdigit = mod(myvar, 10)
. qui forval i = 0/9 {
.         count if lastdigit == `i'
.         local obs "`obs' `r(N)'"
. }
. chitesti `obs'

Saved values

r(k)         number of classes in distribution
r(df)        degrees of freedom
r(chi2)      Pearson chi-square
r(p)         P-value of Pearson chi-square
r(chi2_lr)   likelihood-ratio chi-square
r(p_lr)      P-value of likelihood-ratio chi-square
r(emean)     mean expected frequency

Author

Nicholas J. Cox, University of Durham, U.K.
n.j.cox@durham.ac.uk

Acknowledgements

Benoit Dulong pointed towards a precision problem.

References

Breiman, L. 1973. Statistics: with a view towards applications. Boston:
Houghton Mifflin.

Also see

On-line:  help for chitest, tabulate

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