back-testing
Back testing means checking how an investing idea would have performed in the past before you trust it with real money today.
In plain English
Think of the aunty in Lagos who wants to try a new jollof recipe for a big owambe. Before she cooks for three hundred guests, she makes a small pot first, using the exact measurements, and tastes it against what she remembers worked well at past parties. Back testing in investing is that small pot. It is running your trading idea or strategy against old market data to see how it would have behaved, before you risk actual cash on it.
Say a Nigerian investor has a theory that buying shares of certain banking stocks every January and selling in March has been profitable. Before putting real money in, they would pull five or ten years of price data and check, month by month, whether that pattern actually held up, and by how much. If it only worked twice out of ten times, that theory needs more work, no matter how convincing it sounded over drinks with friends.
In the United Kingdom, a fund manager might back test a strategy of buying FTSE 100 companies whenever their price dips below a certain average, using data going back through several market cycles including the 2008 crash and the 2020 pandemic crash. Those stress periods matter, because a strategy that only looks good in calm markets can fall apart the moment real trouble arrives.
Back testing is not a guarantee of future results, and it is worth saying that plainly. Markets change, economies shift, and past patterns sometimes break for reasons nobody predicted. But it still gives you something more solid than a gut feeling or a tip from someone's cousin. It shows your discomfort or confidence is at least based on evidence, not vibes.
An investor tests a rule of buying MTN Nigeria shares whenever they fall five percent in a week, using ten years of price history. The back test shows this rule would have returned about ₦180,000 profit on a ₦1,000,000 portfolio over that period, though it also shows three years where the rule lost money, which is useful to know before committing real funds.
Questions people ask
Do I need special software to back test a strategy?
Not necessarily. Simple back testing can be done with a spreadsheet and historical price data, which many brokers and financial websites provide free of charge. More advanced investors use dedicated platforms or programming tools, but a determined beginner with Excel and patience can test a basic idea perfectly well.
Can back testing predict what will happen in the future?
No, and this is the most important thing to understand. Back testing only shows how a strategy would have performed under past conditions. It raises your confidence and highlights weaknesses, but markets can always behave in ways they never have before, so no back test removes risk entirely.
What is the biggest mistake people make when back testing?
Testing a strategy on a period that suits it perfectly and ignoring harder times. This is sometimes called cherry picking. A fair back test should include calm markets and rough ones, such as recessions or crashes, so you see how the strategy behaves when things get difficult, not only when they are easy.
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