Old NFO sent me a link to an article proving that the old "apples to oranges" comparison is actually not as silly as it might seem. It's from Improbable Research, the people who bring us the annual Ig Nobel awards.
... it is not difficult to demonstrate that apples and oranges can, in fact, be compared (see figure 1).
Materials and Methods
Both samples were prepared by gently desiccating them in a convection oven at low temperature over the course of several days. The dried samples were then mixed with potassium bromide and ground in a small ball-bearing mill for two minutes. One hundred milligrams of each of the resulting powders were then pressed into a circular pellet... Figure 2 shows a comparison of the 4000-400 cm-1 (2.5-25 mm) infrared transmission spectra of a Granny Smith apple and a Sunkist Navel orange.
Not only was this comparison easy to make, but it is apparent from the figure that apples and oranges are very similar. Thus, it would appear that the comparing apples and oranges defense should no longer be considered valid. This is a somewhat startling revelation. It can be anticipated to have a dramatic effect on the strategies used in arguments and discussions in the future.
There's more at the link, including the figures referred to above.
Just goes to show: never challenge a scientist unless you're willing to live with the consequences!