A familiar workhorse of Internet advertising is the display, or graphical, ad. Roughly a third of all online ad spending goes to such ads, yet Internet users are increasingly reluctant to click on them.

A recent study, by the research firm comScore, has found that the proportion of American Internet users clicking on display ads at least once a month fell to 16 percent from 32 percent over the 20-month period ending in March. And on top of that, just half of those clickers accounted for the overwhelming majority of all clicks on display ads.

That is bad news for the hosting Web sites, which often are paid by the click. But measuring an ad’s success by the click “grossly understates the importance of an advertising campaign,” said Andrew Lipsman, comScore’s director of industry analysis.

Mr. Lipsman said comScore studies had found that merely looking at a display ad increases the likelihood that a viewer will later search for the brand or make a purchase. And many ads are aimed purely at building brand awareness, a form of value that isn’t captured by clicks alone.

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