ByJohn S. McClenahen For more than a decade, analysts have been asserting that companies that have computer networks are more productive than those that don't. Indeed, in an introduction to the Commerce Department's fifth annual report on IT, released this month, Secretary Donald L. Evans repeats the claim. But the evidence to back the assertion is a lot less than it could be. "Estimates of the size of the effect [of IT on economic performance], and how IT makes its impact, remain hard to pinpoint," the report concedes. "Data gaps make it hard to conduct careful analyses on the effect of IT. Continuing efforts by researchers and statistical organizations are filling some of the gaps, but the gaps remain largest for the sectors outside manufacturing -- the sectors that are most IT-intensive."