[Data] When to buy the TomTom app on iTunes

Few days ago, looking for my next holiday in Italy, I started to look around for a TomTom application for my iPhone to buy on the App Store.
I had to choose between the TomTom Italy (34,99 Euro) and the TomTom Western Europe (49,99 Euro): well, for 15 Euros more I decided for the latter, looking forward to go around the Europe later, but, because it was late night I delayed the purchase for the day after.

Surprise Surprise

The few days after I go on the iTunes store to check the app and... surprise, the price jumped to 69,99 Euros! 20 Euros more, wow!
So, discouraged about the thing I started to think that it was all my fault and that probably I should have bought before, when there was the previous promotion (?): but I haven't seen anywhere about a promotion on the TomTom app, how can I figure out there was one?!

So, even if I was still tempted to buy it, I've checked a bit online to find out if TomTom had published any promotion about it's iPhone app... nothing.

Data matters

So, when I was about to buy the app at the new price it comes back to my mind that a site that tracks all the price changes in the App Store, so looking for another promotion from TomTom I gave it a try.

Filtering all the apps (in the right market), I've found this small table, in the corner:

Looking at that table it's easy to see that there's something strange going on with the price: why so many changes in the period?

Collecting and playing a bit with the data I manage to have a representation of the trend of the changes of the app on the store:

Don't you see a strange accelleration of the changes during the last period? If they change so quick the price it might happen a price drop soon, so waiting a bit it might be worth...
Starting from the statement above let's filter out the data by week days and see if I have any chance to buy the app with a lower price:

Mmmmm... still too generic: I can only see that they're changing the price usually during Wednesday and Saturday more often than the other week days. But I cannot see if the change is positive or negative! Luckily Google Spreadsheet can handle multiple criteria so with a formula like:

=COUNT(FILTER(C2:C;C2:C=0;D2:D="Positive"))

The spreadsheet starts to count only the positive and negative change occurrencies in the data for each weekday.*
At the end something more interesting comes out of the data:

More, for the year 2013:

Conclusions

So, because it was Wednesday when the price was 69,99 Euros there's still a small chance to buy the app for 20 Euros less the day after: maybe it was worth to try for a 30% sale...

And there it is: the day after it comes the price drop again and I've got now my TomTom app at a very good price!

This the story on why study the data, sometimes, it's worth. At least 20 Euros.
Now just remember to buy the TomTom app only on Thursdays!

* The Column "C" had the Weekday number (0->6) and the Column "D" was a boolean column filled with the strings "Positive" and "Negative".