We measure distance in meters, mass in kilograms, time in seconds..why couldn't we measure software in some units? This is the question that made room for software metrics as a way to asses the complexity of a given software system.
However, things are not that simple. What would you say a good metric is be for software? Number of lines of code? Man hours? Number of lines of client specifications? Function point analysis?
You can easily see there's no silver bullet for this and all of the above can easily be cheated to give a bigger (or smaller) metric value so finding the perfect metric is still an open problem. There are other problems too: consider that a manager assigns his best programmer to the hardest part of the problem which means it may take the longest time to finish the task and may produce the most defects due to the difficulty of the task. Comparing programmers only from the point of a metric is therefore irrelevant.
