
How To Read an Unlabeled Sales Chart - bslatkin
http://evanmiller.org/how-to-read-an-unlabeled-sales-chart.html
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bbrizzi
The first technique won't work if the sales are rounded off to the nearest
pixel, which is pretty much almost always the case.

The second method won't work if the sales do not follow a Poisson
distribution, for instance a product will have better sales at launch and
during advertising campaigns. The sales figures don't always float around a
constant average.

Also, I hadn't checked the math on the second method but the units don't add
up.

On the LHS, you have [sales / pixel]x[pixels] = [sales]

On the RHS, you have ([sales / pixel]x[pixels])² = [sales]²

IIRC, the left hand side should be squared.

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jules
Both sides are unitless: sales is not in dollars but the number of sales.

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sesqu
The units are actually rates: sales per day.

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willvarfar
The article could have computed the numbers on various charts that others have
published that do have the scale visible, to illustrate how well this works.

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jychang
Surprised no one on HN calculated this already.

S = P / Var(P)

Numbers of pixels: 13,6,16,29,6,16,13,6,19,13,19,9,26; followed by the large
one. Mean ~ 14.692; Using Sample Variance ~ 53.397; Using Population Variance
~ 49.289;

SalesPerPixel(Sample) ~ .275

This doesn't seem right, where does the math screw up?

That said, this isn't very useful. The only time someone will show sales
graphs is for special events... aka when it DOESN'T follow a possion
distribution.

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sesqu
I think you underestimated the pixel counts. It's hard to be precise because
of the bleeding, likely due to scaling, but adding one to the pixel counts
results in a much more reasonable 0.294 sales per pixel. This would indicate
the special event precipitated 25 sales.

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lunchladydoris
"Here's a table with more values (the Riemann zeta, unfortunately, is not
implemented in Excel)"

This made me laugh.

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PeterisP
The article fails to mention the very huge dependency on scale for such a
methods.

These methods will work on a sales graph built from numbers like [27, 41, 55,
73]. Part of these methods (not all) may work on a sales graph built from
numbers like [27 000, 41 000, 55 000, 73 000] ... and give the exact same
result as for [27, 41, 55, 73].

And none of them will give any reasonable results for numbers like [27 123, 41
432, 55 132, 73 433] - there's not enough information for that.

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ars
This assumes that all sales are equal, but if sales is a sum of dollars earned
I assume this won't work (since each sale can be for a different amount).

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glogla
Sure, but then it gives you estimate of dollars earned.

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gus_massa
It will give you a estimate of _cents_ earned, but they are usually too small
to appear in the graphic. In this case the first method doesn't work.

