The Israeli Human Rights group, B'Tselem, has produced a report in which they claim their thorough research has demonstrated that 1,387 Gazans were killed during Cast Lead and that of those 773 "did not take part in hostilities."
As an indication of just how varied these numbers are, the Palestinian Centre for Human Rights (PCHR) suggests 1,419 Gazans were killed in Cast Lead with 1,167 "non-combatants" which includes "...civilians and civil police officers who were not involved in hostilities."
On the other hand, this report from the International Institute for Counter Terrorism finds that at least 564 casualties from Cast Lead were "...casualties affiliated with the organization or one of its units, or who bear a title identifying them as Hamas casualties..." (emphasis in original) but does not take on the question of how many casualties there were in total. Similarly some blogs, like this one, have been doing some of their own research--which though it may be less credible, is not necessarily wrong--that concluded there were "...656 legitimate targets..." amongst the casualties.
The IDF, which has its own statistics, did not issue a response to any of these reports.
This seems to beg two questions, first, what is a "legitimate target" and second, who has their numbers right?
B'Teslem explains that their standards for what a legitimate target was summarized as follows from a Red Cross study of the question:
"1. Persons who fulfill a “continuous combat function.” Such persons are
legitimate objects of attack, even if at the moment of attack they are not
taking a direct part in the hostilities. This category includes persons who are involved on an ongoing basis in the preparation, execution, or command of combat acts or operations. An individual recruited, trained and equipped by such a group to continuously and directly participate in hostilities can be considered to assume a continuous combat function even before he or she carries out a hostile act. On the other hand, persons who continuously accompany or support an organized armed group but whose function does not involve direct participation in hostilities maintain their status as civilians and are not legitimate objects of attack. Thus, recruiters, financiers and propagandists may contribute to the general war effort, but as long as they do not directly participate in hostilities, they are not a legitimate object of attack.
2. Persons who do not fulfill a “continuous combat function” but take a direct part in hostilities (for example, on their way to fir [sic] a rocket, during the firing of the rocket, and on the way back)."
The definition excludes from the list of legitimate targets those involved in political activities or support activities. Of all the statistics cited above, the B'Tselem report seems to be the only one to clearly lay out the standards it used to determine who was a combatant and who was a non-combatant.
Entire books have been written on this question and so it cannot be treated fairly here, but suffice to say that the Red Cross definition sounds reasonable for defining those who are directly involved in hostilities, but it does not treat the question of who is a legitimate target. Certainly, anyone involved in hostilities would be a legitimate target, but what of the political leadership of a group like Hamas? It could be argued that they commanded combat acts. The definition also contains ambiguities, suggesting that financiers are not combatants, but if the preparation for an attack requires fundraising, then it seems financiers actually fall into a grey zone.
Legitimate target, therefore, remains a term subject to an almost case by case definition. As explained here by a former IDF lawyer, in Cast Lead, Israel decided that the threat against it was such that the entire infrastructure behind the threat was a legitimate target, and so it broadened the definition of legitimate target beyond the Red Cross definition of those directly involved in hostilities.
As for which statistics to trust, it seems that with such wide variations and regular sniping--one side against the credibility of the other's numbers--the exact figure may never be agreed upon. In reading the reports (or summaries, as the case may be), however, it seems that B'Tselem offers the most balanced presentation of its statistics, noting significantly that just because more civilians were killed, does not imply a violation of international law. This is a crucial point that PCHR does not seem to consider in its report which is difficult to consider seriously based on the terminology it employs alone (IDF becomes IOF--Israeli Occupation Forces, and Hamas is the "resistance" in PCHR reports.)
This being said, despite what seems to be prima facie superior credibility for B'Tselem (due in no small part to the thorough work they appear to have carried out and the documentation they claim to have), important legal questions remain about whether it is right to include an individual amongst the civilian casualties or illegitimate targets merely because they did not participate in hostilities.