How is a seasonality chart calculated for a specific stock?

External question — Personal Finance Stack Exchange Author: KingsInnerSoul Original post: https://money.stackexchange.com/questions/158689 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. I have seen several websites offer a Seasonality chart for stocks, and I tried to recreate it, but my results have the general line pattern, but its not the same as what I see on other sites. My calculation was as follows: Get the Closing price of the stock for each day since the IPO, Break down the price to annual results - basically arrays for the closing price per day for that year, Create another set of arrays, for each year, where the rate of change is calculated (today's closing minus yesterday's closing), and the first trading day of the year is zero, Create another set of arrays, for each year, where it calculates the percent change for each day, (today's closing minus yesterday's closing) divided by today's closing multiplied by 100 to get the percent, Then for each year, create a running cumulative percent change; add the percent change for each. Create a daily average of the running cumulative change, and then plot it. Issue is that when I compare my result to what other sites are showing, its different. Is my calculation method correct?

External answer — Personal Finance Stack Exchange Author: Bob Baerker Original post: https://money.stackexchange.com/a/158692 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. If your results are different from what is displayed on various web sites then your methodology isn't the same. A quick google search indicates that there are a variety of ways that a generic indicator can be calculated. I'd offer that it isn't that important to duplicate a particular web site's numbers but rather, can you discern anything useful from whatever you're looking at? And some sources already do the heavy lifting - for example, ThinkOrSwim.

External answer — Personal Finance Stack Exchange Author: D Stanley Original post: https://money.stackexchange.com/a/161850 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. A "running cumulative percent" is not the right approach. If a stock goes up by 50% one day and 50% again the second day, the cumulative YTD change is not 100%, but 125% (1.5 * 1.5 = 2.25). That is likely one difference you see. A better methodology would be: Create a 2-d array of price, where the rows are the years and the columns are the days of that year (~260 weekdays) divide all values in each row by the price on the first day of that year (the first element in that row) subtract 1 from everything That should get you a rolling YTD percentage for each year, starting at 0%.