# rolling months

#### Jignesh77

##### Senior Member
The 12-month rolling sum is the total amount from the past 12 months. As the 12-month period “rolls” forward each month, the amount from the latest month is added and the one-year-old amount is subtracted. The result is a 12-month sum that has rolled forward to the new month. Below as an example demonstrated with visual and mathematical. In the example, the business began operations in January 2019. The monthly emissions are totalled...
https://www.pca.state.mn.us/sites/default/files/p-sbap5-34.pdf
What does "roll" mean in "As the 12-month period “rolls” forward each month"?
What does 12 month rolling months mean?

##### Senior Member
Here is an example. I'm going to take a three-month rolling period of the year. It begins with January-February-March and the three-month period rolls forward one month each time:
1. Jan-Feb-Mar
2. Feb-Mar-Apr
3. Mar-Apr-May
4. Apr-May-Jun
5. May-Jun-Jul
6. Etc.
"12 month rolling months" doesn't appear in your quote, does it? Do you mean "The 12-month rolling sum"? That would be the sum of whatever it is (monthly profits, Covid deaths, Sonu Nigam records sold...?) for the period in question.

#### Jignesh77

##### Senior Member
"I want to aggregate data for rolling 12 months antibiotic items per 1000 patient count under 10 at ccg and at national level so I can produce a three line chart showing practice, CCG and national trend over time.
So I need to aggregate

rolling 12 Month rolling items (at CCG and national)

divide by

aggregate population 0-9 (at CCG and national level)

and multiply by

1000"
I saw the above in one of the Forums for analysing prescribing data. What does "rolling 12 months" mean here? what does "roll" mean exactly? Sorry, to ask again as I am still confused.

#### Uncle Jack

##### Senior Member
"Rolling" (adjective) is only used like this in a few very specific situations, generally related to finance, business, and certain types of statistics, where data is presented for each month, or day, or some other time period, but the data covers a longer period of time than just that month or day. A rolling 12-month total is a typical example. The rolling 12-month total for December is the total for the entire year, from January to December. The rolling 12-month total for January is the total for February to December of the previous year, plus January of the current year. Sometimes the months or days are offset by one, so a rolling 12-month total for January refers to the total from January to December of the previous year, and the rolling 12-month total for February refers to the total from February to December of the previous year, plus January of the current year.

When you move from one month to the next, rather than adding together the values for each of the twelve months, you could take the previous month's 12-month rolling total, subtract the first month's value and add the next month's value. So, if the previous 12-month rolling total was for January to December of the previous year, you would subtract the value for January of the previous year and add the value for January of the current year. This is the method your quote describes, and they call this rolling the sum forward to the new month, using "roll" as a verb. As far as I am aware, "roll" is not usually used as a verb like this, and it is probably only ever used like this to describe this meaning of "rolling" (adjective).

##### Senior Member
I agree with Uncle Jack. I was already preparing to explain that your rolling 12-month periods are, say:
1. Jan 2018-Dec 2018
2. Feb 2018-Jan 2019
3. Mar 2018-Feb 2019
4. Apr 2018-Mar 2019...
They "roll forward" by deducting the first month and adding the next month following, each time. So each period is still 12 months long.

The advantage of this method of showing figures is that it smooths out sudden jumps and seasonal trends, giving a more realistic display of long-term trends. So it's also known as "smoothing".

If you want to see the effect, look at an arbitrary example on Google Books Ngram Viewer. That shows that the word "blimp" was suddenly used more frequently in the period 1940-1942. But if you adjust the "smoothing" box, it gradually shows the longer-term trends instead of the annual ones.

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#### numerator

##### New Member
I think the usage started out as a metaphor - the evaluated block of time is moving smoothly forward, as if it was rolling on wheels. This is in contrast to eg. evaluating by calendar year, i.e. always Jan to Dec, Jan to Dec...

My language (Slovak) uses the word "sliding" for the same concept - perhaps that suits your imagination better? Unfortunately the metaphor has ossified in English and you are not given a choice of metaphor - you must use "rolling".

##### Senior Member
I see it as the image of a vehicle on caterpillar tracks. Here there are about 12 plates on the ground at any one moment. Every time a new plate hits the ground at the front, an old one lifts off at the back. The entire structure thus rolls forward, always with 12 units touching the ground. The very image of a 12-month rolling period:

#### kentix

##### Senior Member

Well done. Just write the names of the months on them and you've earned the A+.

The point is calculations are made for a whole year (i.e. 12 month period) but not a calendar year, not January to December. It's 12 months counting backwards from today/this month.

If the calculations are done on June 1, 2020, it includes May 2020 back to and including June 2019.

#### Uncle Jack

##### Senior Member
Quite honestly, I think you can simply ignore the verb "roll". This use of "rolling" as an adjective is common, but I don't think I have come across "roll" as a verb used in this way before.