Is it routine for historical stock price data to be revised?

External question — Personal Finance Stack Exchange Author: regularguyprobably Original post: https://money.stackexchange.com/questions/150399 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 noticed that the price history of AT&T stock (T) prior to April 11, 2022 is about 25% lower today than it was a week ago. That is to say that when I click on my favorite financial websites and view the price history, viewed on April 8, 2022 I see it trading around $23 that day. Viewed on April 15, 2022 I see April 8, 2022 trading around $18. I noticed because I purchased shares prior to that date, and I know how much I paid and when. The date of change (April 11, 2022) seems to coincide with the completion of a merger and I guess that it is somehow related, though it isn't obvious to me why or how this could change history. Is it routine for historical data to be changed as a result of a merger? Excluding splits, are there any other events by which historical data may be altered? Who benefits by this alteration of historical data? Is there a mechanism by which investors may easily view the date and magnitude of such adjustments (the raw data)? What, if any, events would cause the historical data to be revised upward? edit: questions 4 and 5 added

External answer — Personal Finance Stack Exchange Author: Flux Original post: https://money.stackexchange.com/a/150401 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. Is it routine for historical stock price data to be revised? Yes. In fact, this happens not just for mergers, but also for dividends, stock splits, and spinoffs. Excluding splits, are there any other events by which historical data may be altered? The most common occurrence is when a company pays a cash dividend. If you are looking at a dividend-adjusted price chart, then its historical prices have been adjusted to remove the sudden drops in prices on the ex-dividend dates. Who benefits by this alteration of historical data? There are uses for raw prices (unaltered data), but for the vast majority of people who use price charts for analysis, the adjusted data is more useful because it makes the current data more comparable to the past data. For example, if a company does a large spinoff, its stock price will decrease. If you only look at the raw prices, it would seem that the investors lost a lot of money from the "crash". The price adjustment is meant to present a more accurate picture of investment performance. Formulas used for adjustments: CRSP Calculations Nasdaq Data Link Blog: The Comprehensive Guide to Stock Price Calculation What, if any, events would cause the historical data to be revised upward? A reverse stock split (a.k.a. share consolidation) would cause the historical data to be revised upwards. Refer to the adjustment formula and example calculation in the link above (section: "Reverse Stock Splits"). The date of change (April 11, 2022) seems to coincide with the completion of a merger ... What happened was that AT&T spun off a subsidiary (WarnerMedia), and merged that subsidiary with Discovery Inc. to create Warner Bros. Discovery Inc. Sources: AT&T Announces Details for Completion of Spin-Off Ahead of Close of WarnerMedia Transaction Discovery and AT&T Close WarnerMedia Transaction

External answer — Personal Finance Stack Exchange Author: Bob Baerker Original post: https://money.stackexchange.com/a/150411 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. There's nothing nefarious going on here. When a company does a spin off, the shareholder receives shares in the spun off company. The share price of the parent company drops by the value of the new company. While this was part of a merger, the spin off was a separate action. Yes, it is routine for historical data to be changed as a result of a spin off. Share price is also adjusted for common as well as special dividends. There is no immediate benefit to the spin-off. It's a zero-sum event. The magnitude of the data adjustment is known from the public information detailing the terms of the spin-off. It can also be viewed in the historical data where adjusted share price info is provided (for example, see Yahoo Finance).