## Wednesday, February 17, 2010

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### It increased by how much?

In yesterday’s meetings, someone presented some data, showing us rates of sending spam, month by month, broken down by country. The summary page showed the change from six months ago to last month. Here’s an example of the summary table, with made-up numbers:

Spam by country, as % of total
CountryJul 09Jan 10Change
Slobovia4%8%+4%
Gumana2%< 1%-1%

Maybe you already see the problem with this.

A change from 4% to 8% does not represent a 4% increase. It’s a 100% increase — the spam from Slobovia has not gone up by 4% in six months, it’s doubled. And similarly for the other numbers: Parador showed a 56.4% decrease.

What is true is that Slobovia’s contribution to the total has increased by 4 percentage points (which is not the same as saying that it’s increased by 4 percent). And if it’s clear that that’s what you’re saying, it’s a fine thing to say... we can then debate which number is more useful, and the answer to that will depend upon what we’re using the numbers for.

But it’s very misleading to list those numbers in a table like that, and entirely wrong to say that “the amount of spam from Slobovia has increased by 4% since last July.”

Gotta be careful with percentages.