Why RSI Values Differ Across Trading Platforms
Summary
This note explains why a custom Relative Strength Index may not match the value shown by a charting platform, even when both display the same nominal period. Potential causes include different smoothing methods, different amounts of historical price data used to initialize or calculate the indicator, and differences in data vendors’ adjusted price histories. Wilder’s original approach uses the full available series, while some implementations use a limited lookback or substitute another smoothing variant.
A suggested diagnostic is to compare calculations on a security from its initial public trading date, reducing uncertainty about hidden earlier history. This can help isolate history-length effects but does not prove that two platforms use the same formula or price adjustments. The note offers no controlled comparison or general rule for reconciling outputs; identifying the precise data and algorithm conventions is necessary to explain a particular discrepancy.
Key ideas
- RSI readings can differ because platforms use different smoothing or initialization methods.
- Different available price histories can change the indicator’s calculation.
- Corporate-action adjustments to historical prices may also produce different values.
- Starting a comparison at an IPO can help isolate hidden-history differences, but cannot resolve every cause.
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# Why there is a Deviation on my RSI indicator in comparison from one of the other Trading Markets # Why there is a Deviation on my RSI indicator in comparison from one of the other Trading Markets Hello all, and sorry for this stupid question. I am using my custom RSI indicator to which I created programmatically. I follow the equation type from here. My problem is when I compare it with the Trading view RSI indicator from TradingView (of course, for the same period). I have noticed many deviances sometimes. Lets take a look in an example. Let's assume in a specific time xx:xx, my custom indicator says RSI is `30.2103` and TradingView RSI says `29.8654`(of course, for the same period). Why does this deviation happen? Could someone explain this to me, because I ave been struggling for so many hours to understand this problem and I would appreciate your help if you have the appropriate knowledge in this field. Hello all, and sorry for this stupid question. I am using my custom RSI indicator to which I created programmatically. I follow the equation type from here. My problem is when I compare it with the Trading view RSI indicator from TradingView (of course, for the same period). I have noticed many deviances sometimes. Lets take a look in an example. Let's assume in a specific time xx:xx, my custom indicator says RSI is `30.2103` and TradingView RSI says `29.8654`(of course, for the same period). Why does this deviation happen? Could someone explain this to me, because I ave been struggling for so many hours to understand this problem and I would appreciate your help if you have the appropriate knowledge in this field. ## Answer by Richard at NorgateData (score 6, accepted) https://quant.stackexchange.com/a/39854 J. Welles Wilder Jr created the indicator called the Relative Strength Indicator in 1967. The indicator he originally created uses all data points in the sample series, not just the last 14 data points (or whatever period of RSI you are using). Any data series that has less (or more) data than your current set will therefore show a different set of data. Many charting/analysis packages use a limited lookback for RSI to simplify their algorithms. Also many data sources have longer data history than others. Some analysis/charting software/sites simplify the RSI by replacing Welles Wilder's smoothing with their own variant. They may also restrict the historical period of calculation of the indicator, presumably for efficiency reasons. Lastly, the historical price series in one source may have been adjusted for any of the following events: splits, reverse splits, spinoffs, corporate restructures, stock dividends, long term capital gains, short term capital gains, return of capital, special dividends, or ordinary dividends. Unless you are able to determine the exact price adjustment methodology and RSI algorithms used, trying to figure out why one RSI is different to another is an exercise in futility. ## Answer by ehiller (score 0) https://quant.stackexchange.com/a/78934 One technique I've used to narrow down differences in methodology by the various trading platforms is to use IPOs (starting at the IPO date), this way I am guaranteed that there is no historical data being loaded behind the scenes. As I write this in April 2024, I have been using ANRO which had its first trading day on 2024-02-02. I hope this helps you or others in the future, it doesn't always reveal differences, but many times it has helped and at the very least it eliminates one possible variable.
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