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From |
Garry Anderson <[email protected]> |

To |
[email protected] |

Subject |
st: Confidence interval of difference between two proportions and -csi- |

Date |
Thu, 18 Mar 2004 16:37:54 +1100 |

Hi,

I am enquiring if a more appropriate method could be used please to calculate the 95% CI of the difference between two proportions in the -csi- command?

At the moment it is possible for the upper bound of the confidence interval of the difference between two proportions to be greater than 1.0. I realize that the approximation that is used is not appropriate for small sample sizes, however I think that reporting of results that are impossible should be avoided.

For example

-csi 5 1 0 4,e-

| Exposed Unexposed | Total

-----------------+------------------------+----------

Cases | 5 1 | 6

Noncases | 0 4 | 4

-----------------+------------------------+----------

Total | 5 5 | 10

| |

Risk | 1 .2 | .6

| |

| Point estimate | [95% Conf. Interval]

|------------------------+----------------------

Risk difference | .8 | .449391 1.150609

Risk ratio | 5 | .866228 28.86076

Attr. frac. ex. | .8 | -.1544305 .9653509

Attr. frac. pop | .6666667 |

+-----------------------------------------------

1-sided Fisher's exact P = 0.0238

2-sided Fisher's exact P = 0.0476

The value of 1.15 is not desirable.

One potential solution to this is to use a method that is described in

Newcombe RG (1998) Interval estimation for the difference between independent proportions: comparison of eleven methods. Statistics in Medicine 17: 873-890

The -ci- command gives an exact 95% CI for a proportion, therefore I think that it is desirable that the difference between two proportions have sensible 95% confidence intervals.

Kind regards, Garry

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