Three questions, three kinds of tool.

Why the third question is back. Attribution works by following one person from ad to purchase. Privacy laws and phone makers now stop much of that tracking. Marketing-mix modelling does not follow anyone. It looks at totals, so it still works.

Reports. A marketer runs campaigns on many channels, and each channel reports its results in its own way. Some tools bring all this data into one set of reports.
How the money flows. Advertisers pay these tools a subscription, or a fee for each thousand ads checked. The tools must be independent of the platforms that sell the ads, so that advertisers trust their numbers.
Every company named here has posted software engineering jobs in India. Famous companies that don't actively hire software engineers in India are left out.
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Continue with Google- Tools that check and measure ads
Reports and measurement of effect
Attribution in apps
Verification
Other companies
Integral Ad Science and AppsFlyer are not covered in this Market Segment. Nielsen, which measures how many people watch TV and streaming, is covered in another Market Segment.
Attribution was built on following one person from ad to purchase. Privacy rules and phone makers have ended much of that. AI models are bringing back the older, statistical method, which measures what spending caused across a whole market. They also make it fast enough to steer campaigns every week, rather than once a year.
Who these companies hire, and for what, is on What adtech companies hire for.