What is the Impact and Difference Between IMG and JS View Tags?
C3's JS (JavaScript) view tag is a two-way tag. It can essentially speak back and forth to a page, and the key function among other things for C3's JS view tag is to determine viewability--if the display ad is in view per the IAB guidelines.
IMG tags are basically a one way tag, not a two way tag. There are certain cases where only an IMG view tag can be deployed: PreRoll video, In-App mobile, and a small number of publisher sites which is very rare.
IMG tags are of ‘undetermined view status’. C3 provides credit to these since it's not fair to say these ads are not in view because they may indeed be in view. Essentially we give them the benefit of the doubt.
For PreRoll video, nearly 80% of PreRoll video ads are completed in the industry, and as soon as the ad starts, our C3 IMG view tag loads and once loaded we deem the PreRoll video ad to be in view. In reality, perhaps 20% don't complete viewing the entire 15 or 30 second ad...but the IAB guideline is met regardless. In addition, for YouTube, consumers can "Skip this Ad" in the first four seconds, and as a result, YouTube subtracts this view impression from its impression counts, while C3 Metrics still keeps it, and sometimes C3 will show higher impression counts versus YouTube for this reason.
For a rare list of publisher sites not taking the common JS view tag, C3 gives those sites the benefit of the doubt with the undetermined status, assuming they were in view. If these sites are typical publisher sites displaying 300 x 250's, 728 x 90's, and 160 x 600's it is most likely that display units shown in this environment have the same viewability percent as the rest of your display campaigns deploying C3's JS view tag.
In the case, what happens is false positive to the tune of 70%. So for regular display, this represents a big issue delivering false positives, handicaps the other networks doing the right thing with a JS view tag, and crowds out attributed value for those delivering the JS view tag.
What C3 can do in this case is decrement the IMG tag network ID's attributed value by your "not viewable" percent in our admin back end as a rule, but it still does not solve the issue that an IMG tag display unit will have 4x the likelihood of securing Originator, Roster, Assist, or Converter position even if the value is decremented by a setting in the back end.
Thus, in a normal publisher display environment, a JS view tag is always the go-to preferred method to ensure the highest accuracy.