Heterogeneity is a statistical term referring to the diversity of the population averaged to derive the disability statistics.  We agree that the population of those with a specific impairment is quite diverse.  However, it is incorrect to surmise that this precludes the use of a statistic drawn from this population.  In fact, two studies have concluded that the type of limitation has little, if any, impact on employment (Bolton, Bellini, & Brookings, 2000; MacDonald-Wilson, Rogers, & Massaro, 2003). 

We further agree that any statistic applied should be applied appropriately.  In cases concerning the assessment of lost worklife, the expert should rely on medical reports verifying the impairment and apply the appropriate "vocational scrutiny" to assess the impact of the impairment, if any, on the individual's ability to work.  The critical vocational issue is whether a medically determinable permanent physical or mental impairment exists that limits or is likely to limit the individual in terms of the amount or kind of work he or she is capable of performing.  Work disability data from the CPS regarding employment should be used only if the individual has a work disability regardless of the severity of the impairment. 

The solution for some critics of disability statistics is to use worklife statistics that are not specific to disability at all.  It is clear that this is not a reasonable solution as it uses statistics from a much broader and more diverse population than the disability specific worklife expectancies.  Most importantly, it ignores what is known about persons with a disability.  They experience lower levels of employment and, hence, a reduction in worklife expectancy. 

When predicting the height of a 5-year-old boy, should one use an average of all people or of 5-year-old boys?  Similarly, when predicting the employment experience of, for instance, a female with a nonsevere work disability, should one use an average of all people or of females with a nonsevere work disability?

Most forensic economists routinely project earnings and employment based upon education.  Categorization by education results in averages from diverse populations.  Imagine, for example, averages for persons with a baccalaureate degree.  These degrees may range from an Accountant/CPA or Mechanical Engineer to a Social Worker, or lead to occupations ranging from a CEO of a large corporation to a minister.  Obviously, these occupations have a wide range of expected earnings.  Yet, despite the diversity, this average is accepted and used by many economic and vocational experts, as it is the most appropriate predictor in many cases – such as a college freshman who had not yet determined an area of focus.  Allowing use of a broad education average while not allowing a more narrowly defined disability-specific education average is a nonsensical double standard.