Possible Credit Systems
From Unofficial BOINC Wiki
There are a lot of valid ways that the contribution can be measured. A few of them are:
- A count of Work Units processed.
- A count of CPU Time applied in seconds.
- A count of the mathematical operations performed by the CPU.
- A count of CPU Time used in seconds multiplied by a metric of work production also measured in seconds.
One of the easiest and simplest methods is to simply count the number of Work Units processed. This method has the drawback of being only completely valid and consistent when the Work Units are all identical in size and take the exact same amount of processing resources to complete. Since the BOINC Client Software is a multi-project capable system the simplistic "increment the bean" (for the bean counters, naturally) is immediately invalidated.
You can't count Work Units processed across Projects because of the levels of complexity and processing times will not be the same across projects. (Those concerned only with credits would pick the Projects with the shortest Work Units.)
Counting wall clock time applied is also right out the window because each type of CPU performs a different amount of work in the same number of seconds because of differences in internal architecture and clock speeds. (Slower computers would get more credit than faster ones for the same Work Unit.)
Counting the number of mathematical operations is also right out because we are now wasting a lot of CPU Time just tracking what we are doing with each mathematical operation. Besides, one computer may have built-in capabilities that another computer's processor does not have.
 BOINC Credit System
BOINC uses the method of counting CPU Time and multiplying by a metric of work production in relation to a Reference Computer. This method was chosen due to it's low overhead with reasonable accuracy. Combined with the fatal flaws in the other options mentioned above.