Yahoo Finance Data in RealTest: What Free Data Can and Cannot Test

RealTest imports Yahoo Finance data natively. It is one line in an Import section, it costs nothing, and it is a legitimate way to start. It is also the wrong data for about half of what people then try to do with it, and the failures are not obvious, so it is worth knowing where the line sits before you build a month of work on the wrong side of it.

The import

Import:
	DataSource:	Yahoo
	IncludeList:	SPY, EFA, EEM, AGG, TLT, GLD, IWM
	StartDate:	2007-01-01
	EndDate:	Latest
	SaveAs:	rotation.rtd

Run that once with the -import flag and RealTest writes a data file your strategies then read. Nothing else in a script changes because of where the data came from, which is the useful part. A strategy developed on Yahoo data runs unmodified on Norgate data later, so starting free costs you nothing in rework.

What you can honestly test with it

Yahoo data is fine when the instruments you trade still exist and you already know which ones they are. That covers more than beginners expect.

• ETF rotation and asset allocation. A fixed basket of large ETFs, ranked monthly, is exactly the case Yahoo handles well. The instruments are liquid, long-lived and unambiguous.
• Single-instrument systems. Anything on SPY, QQQ, IWM, GLD or TLT, including the crossover and mean reversion families.
• Index-level work. Symbols such as ^GSPC and ^VIX are available and behave as you would expect.
• Learning the software. Import syntax, strategy structure, exit logic and reading the trade list are all identical regardless of data vendor.

If your strategy is a rules-based system on a handful of ETFs, free data is not a compromise. It is the correct tool and the paid subscription would tell you nothing new.

What breaks

Delisted companies do not exist

When a company is acquired or goes bankrupt, its ticker stops returning history. Backtest a stock strategy on Yahoo and every name in your universe is a company that made it to today, which inflates the result by a margin that grows with the length of the test and shrinks with the size of the companies. This is the single reason stock universe testing on free data does not work, and no setting fixes it. We went through the mechanics in the five backtesting mistakes that fake an edge.

There is no index membership history

Yahoo will not tell you which companies were in the S&P 500 in 2011. That means a filter like RealTest's InSPX has nothing to read, so any strategy defined as trading members of an index cannot be expressed properly. You are left approximating a universe with a list you assembled today, which is the same bias arriving through a different door.

Adjustments move under you

Yahoo supplies prices adjusted for splits and dividends, and those adjusted series get revised as new dividends are paid. A backtest run today and the same backtest run next quarter can return different numbers on identical rules, because the price history itself changed. That is manageable once you know it happens, and confusing for a long time if you do not.

The related trap is mixing adjusted and unadjusted prices in one decision. A stock that split two for one shows a $50 price in an adjusted series where it actually traded at $100, so a rule containing an absolute price threshold means something different in every era of the test.

Reliability is best-effort

Yahoo does not publish a supported public API. The endpoints that serve the website still work and every downloader relies on them, but they change without notice, rate-limit large requests, and sometimes return a missing-data error for a symbol that is trading normally. For an import of a dozen ETFs this is a non-issue. For a nightly refresh of several hundred symbols it becomes a maintenance job.

The honest line

When to stop using it

The trigger is not a date, it is a change in what you are asking. The moment your strategy selects from a universe rather than trading a named instrument, free data stops being able to answer the question. That is the point where Norgate or a comparable provider becomes the cost of getting a real answer, and the relevant tier is the one that includes delisted securities.

Between those two states there is a useful habit. Prototype on Yahoo because the loop is fast and free, then re-run the same script on paid data before any capital is involved. If the result survives the change, you learned that your edge was not made of survivors. If it collapses, you learned that for the price of a subscription rather than a drawdown.

How to start

Import six or seven ETFs first and check the data before writing any strategy. Look at the earliest and latest bar for each symbol, since Yahoo histories start at different dates and a universe where one instrument begins in 2013 will silently change what a monthly rotation does over its first years. Then write one simple system and read the trade list rather than the summary.

If you want a working script to start from rather than a blank file, our free RealTest strategy costs nothing and runs on free data, which makes the whole first pass a zero-cost exercise. What that leaves you with is the RealTest workflow in your hands and a clear view of which question you are asking next.