The routine may be called by the names g07abf or nagf_univar_ci_poisson.
Given a random sample of size , denoted by , from a Poisson distribution with probability function
the point estimate, , for is the sample mean, .
Given and this routine computes a confidence interval for the parameter , denoted by , where is in the interval .
The lower and upper confidence limits are estimated by the solutions to the equations
The relationship between the Poisson distribution and the -distribution (see page 112 of Hastings and Peacock (1975)) is used to derive the equations
where is the deviate associated with the lower tail probability of the -distribution with degrees of freedom.
In turn the relationship between the -distribution and the gamma distribution (see page 70 of Hastings and Peacock (1975)) yields the following equivalent equations;
where is the deviate associated with the lower tail probability, , of the gamma distribution with shape parameter and scale parameter . These deviates are computed using g01fff.
Hastings N A J and Peacock J B (1975) Statistical Distributions Butterworth
Snedecor G W and Cochran W G (1967) Statistical Methods Iowa State University Press
1: – IntegerInput
On entry: , the sample size.
2: – Real (Kind=nag_wp)Input
On entry: the sample mean, .
3: – Real (Kind=nag_wp)Input
On entry: the confidence level, , for two-sided interval estimate. For example gives a confidence interval.
4: – Real (Kind=nag_wp)Output
On exit: the lower limit, , of the confidence interval.
5: – Real (Kind=nag_wp)Output
On exit: the upper limit, , of the confidence interval.
6: – IntegerInput/Output
On entry: ifail must be set to , or to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of means that an error message is printed while a value of means that it is not.
If halting is not appropriate, the value or is recommended. If message printing is undesirable, then the value is recommended. Otherwise, the value is recommended. When the value or is used it is essential to test the value of ifail on exit.
On exit: unless the routine detects an error or a warning has been flagged (see Section 6).
6Error Indicators and Warnings
If on entry or , explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
On entry, .
On entry, .
On entry, .
When using the relationship with the gamma distribution the series to calculate the gamma probabilities has failed to converge. Both tl and tu are set to zero. This is an unlikely error exit.
An unexpected error has been triggered by this routine. Please
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.
For most cases the results should have a relative accuracy of where is the machine precision (see x02ajf). Thus on machines with sufficiently high precision the results should be accurate to significant digits. Some accuracy may be lost when or is very close to , which will occur if clevel is very close to . This should not affect the usual confidence intervals used.
8Parallelism and Performance
g07abf is not threaded in any implementation.
The following example reads in data showing the number of noxious weed seeds and the frequency with which that number occurred in subsamples of meadow grass. The data is taken from page 224 of Snedecor and Cochran (1967). The sample mean is computed as the point estimate of the Poisson parameter . g07abf is then called to compute both a 95% and a 99% confidence interval for the parameter .