Zeiss Humphrey II-i series, User Manual

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Humphrey Field Analyzer II-
i
series User Manual 2660021145640 A
STATPAC Analysis & Printing 7-13
level of less than 1%. This is a slope magnitude on the order of more than thirty times the rate of
change due to aging in the normal population. A slope of only one or two tenths of a decibel per
year would be viewed with considerably less concern, as it is similar in magnitude to the age
correction which has already been applied to the data.
If the hypothesis that the slope is zero is not rejected, that is, that there has been no change in the
patient’s visual field, then the message NOT SIGNIFICANT appears. This indicates that the slope was
not significant at the largest “p” value STATPAC is programmed to consider, 5%.
The larger the number of tests analyzed, the more easily the small changes in MD are detected. A
low number of observations involves a higher risk that the analysis will fail to detect a deterioration
over time. This is the reason that STATPAC will not perform an analysis on fewer than five test
results.
The application of the linear regression analysis means that the following assumptions have been
made:
1. The true MD changes linearly with time.
2. The differences between the measured and the true MD are independent, and identically and
normally distributed.
The STATPAC linear regression of mean deviation, accompanied by the message “MD Slope
Significant” or “Not Significant” appears on the Change Analysis printout as well as on the Guided
Progression Analysis printout.
On the Change Analysis printout, STATPAC modifies this regression analysis whenever marked
learning effects are present. When at least five tests are chosen for analysis, the modified STATPAC
regression analysis discards the first test result in a series if its mean deviation is significantly out of
line with, and worse than, the trend shown in later tests (p < 5%).
On Statistics and Probability
When considering the probability statements in this statistical package, it is important to be
conscious of what they do and do not mean. They are an aid to interpretation, not a diagnosis. The
doctor’s judgement is still the most important element in determining the clinical significance of
perimetric findings.
The probability statements are based on the distribution seen in the normal population. Saying that
less than 5% of the normal population deviates from the norm by a certain amount means just that
and nothing more. It does not mean that there is only a 5% chance that the result is normal.
The positive predictive rate depends, of course, on the prevalence of defective fields in the
population studied. The probability that a given result is abnormal depends on the relative
prevalence in the population of defects caused by disease versus the prevalence of the same field
“defect” in normals. If a certain field result is seen 5% of the time in normals, and similar
glaucomatous field defects are seen in 0.5% of the population, then the result is ten times as likely
to be associated with normality as with disease.
Certainly one should also be aware that some patients commonly seen in a clinical practice may not
meet the criteria of normality (for example, visual acuity) which had to be applied in creating a