# Help Please with Math Question! I was asked this question in an interview and struggled to find an immediate answer

(Last Updated On: September 22, 2011)

### Help Please with Math Question! I was asked this question (see description) in an interview and struggled to find an immediate answer, so I was hoping someone in this group could help me.

I was hesitant to compare \$/user ratios because the sample size is different for each site. I figured you’d have to weight each sample size properly in order to have a fair comparison. For example, let’s say I ended up with 700 clicks for Facebook and resulted in \$1700. Then, it would be easy to conclude that Facebook is more worth investing in online ads.

What techniques/methods would you utilize to conclude which site is more cost efficient at producing the higher number of sales?

Any help is appreciated. Thank you!

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Interesting question — the interviewer gave you one piece of information you didn’t need, and omitted one piece of information you did.

I believe the number of clicks is irrelevant… a “win a free iPod!!!!” banner ad might generate a lot of clicks, but when they arrive at the website of, say, an architectural design firm, probably not a lot of them are going to opt for a new addition to be built on their house.

So you should focus on the profit of Google = \$1,500 vs. Facebook = \$1,200.

The omitted piece of information is how much the advertisement cost — because money spent in one place is not money spent in another. Let’s say the Google ad cost \$10,000 and the Facebook ad cost \$4,000.

Then Google’s return is: (10,000 + 1,500) / 10,000 = 1.15. In other words, for every \$1 invested in the Google ad, you will receive \$1.15 in return.

Likewise, Facebook’s return is (4,000 + 1,200) / 4,000 = 1.30 (earning \$1.30 for every dollar invested).

Of course if you have other goals in terms of needing a particular volume of sales, improving brand recognition, or gaining a presence in a particular new market you might use other metrics to gauge your success.

Hope this helps and good luck with the job search!

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