Chapter 24. Chapter 24 Graphic Content

The take-home message of these graphs is that sterilization seems to increase longevity in male mammals. In order to have greater confidence in this take-home message, it would be helpful to know whether there were similar changes in causes of mortality in cats and humans following sterilization. For example, if rates of death from cancer decreased similarly, that would support the take-home message while suggesting a possible mechanism for this effect.

The graphs use different scales because humans live much longer than do cats, whether they are sterilized or not. Because both graphs have an x-axis that begins at 0, however, it is possible to evaluate the proportion of increase in life span among sterilized males relative to non-sterilized males. If the human graph’s x-axis went from 50 to 70 years, the magnitude of the increase would appear much more dramatic. That would be misleading because it would suggest that the proportion of the increase in longevity among sterilized males relative to non-sterilized males was much greater than it actually is.

Sterilized male cats lived about 8 years as opposed to 5 years for non-sterilized cats—a 60% increase in longevity. For humans, sterilized males lived about 70 years while non-sterilized males lived about 55 years—a 15/55, or 27%, increase in longevity.

Standard deviation is a measure of how much variation in the measure there is from one individual to another.

For populations with the same average value for a trait, in the population with the larger standard deviation individuals would be more likely to deviate from the average. In the population with the smaller standard deviation, individuals would be clustered much more closely around the average. For example, imagine two populations each having an average value of 5 for a trait. If one population has a standard deviation of 0, everyone would have the same value for the trait: 5. If a different population had a very high standard deviation, there could, for example, be many people with measures of 8 or 9, and these individuals would be balanced out by others with measures of 2 or 1, respectively, so that any two individuals would be more likely to differ for the measure (and to differ by a greater amount).

A median, like a mean (which is the same thing as an average), is a measure of the central tendency of a group of measures. The mean is the arithmetical average value, obtained by adding all the measures and dividing by the number of measures. The median is the number for which half of the measures are greater and half are less. The median is less influenced by extreme measures.

The 25th percentile is the number for which 25% of all the individuals fall below (and for which 75% of the individuals are above). The 75th percentile is the number for which 75% of all the individuals fall below (and for which 25% of the individuals are above).

The data from the lower graph were collected from sanitarium patients because at one time, many patients were castrated. The practice is now considered extremely unethical and is not performed. It is extremely rare for any individual to be castrated and it would not be ethical to coerce anyone to undergo castration. The data therefore were available only because the practice once occurred in sanitariums. This experiment certainly could not be conducted today.