In this study we have employed three kinds of incidence:
1. Ever-never. This is the simplest form of incidence, telling whether a person ever or never had a particular experience. For example, 76 per cent of the control group had premarital coitus.
2. Accumulative. This form of incidence tells one what percentage of individuals ever had a particular experience by a given age. For example, 72 per cent of the control group had premarital coitus by age twenty.
3. Age-specific. This form of incidence tells what percentage of individuals experienced a particular activity within a given period of time. We have chosen five years as the period of time and have labeled these years in terms of the person’s age. Thus, after the initial age-period of puberty-15, we have five-year age-periods such as 16-20, 21-25, 26-30, etc. There are, however, a number of important exceptions to this generalization. For instance, we use puberty as a beginning point of adult life, and since puberty is attained at varying ages our first age-period, puberty-15, may contain fewer or more than five years. In the rare cases where puberty is reached after fifteen, our usual second age-period, 16-20, may contain fewer than five years. A termination point in an age-period occurs at the time the person was interviewed: a man interviewed at age thirty-eight obviously has not lived all five years of age-period 36-40. Thus a man’s last (or current) age-period is usually less than five years in duration.
In addition to these time distortions at the beginning and end of a life span, there may be others if the person has been in an institution. We have in this study ruled out the years of life spent in prisons and mental institutions; hence, part or all of an age-period may be deleted. For example, a man who between thirty-one and thirty-five spent three years in a prison has only two years of noninstitutional life in that age-period, but we treat these two years as if they were five. In brief, we assume that had he not been imprisoned the three years would have not been importantly different from the two years he actually lived outside of prison. This assumption is easily defended in cases where the man was «out» at ages thirty-six and thirty-seven and imprisoned from thirty-eight to forty. However, the assumption is in greater danger of error when the man was imprisoned from thirty-six to thirty-eight and «out» from thirty-nine to forty—here the assumption overlooks the possible effects of prison on postprison adjustment. Since we encounter all possible combinations of prison and nonprison life within and overlapping our five-year age-periods, it has been deemed impractical to attempt at this time anything beyond what we have done.
In order to be counted in an age-period a person must have lived six months or more in that period. For example, a man aged thirty-six years and five months is retained in age-period 31-35. whereas a man who has lived thirty-six years and six months graduates into age-period 36-40.
In both accumulative and age-specific incidence, as well as in many of our presentations of frequency, we use five-year age-periods that end on a quinquennial year as follows: puberty-15, 16-20, 21-25, 26-30, etc. These class intervals do not agree with those used by the census and many others, who prefer to begin their periods with a quinquennial year, for example: 15-19, 20-24. Neither method has any great advantage over the other; we have retained our method only because our raw data are often expressed in this form.
Incidence, as we currently define it, concerns the presence or absence of a particular activity regardless of whether that activity resulted in orgasm.
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