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Timing the market is a mug's game (unless you're a professional discretionary trader in which case you have reams of analysis and you're anyway doing it with someone else's money).
The only general-purpose way to get a reasonable return that isn't luck, inside knowledge or very careful analysis is cost averaging, which just means buying in monthly or whatever so you have a good chance of buying close to wherever the bottom turns out to be.
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is cost averaging
I seem to remember reading some analysis which showed that in the majority of time segments, investing a lump sum provided higher returns than PCA, ie time in the market > timing the market.
However many people may not have a lump sum to invest, so it may be a moot point in any case.
My vanguard stocks and shares ISA is almost back to the value it was when I set it up with a lump sum over a year ago. Retrospect probably would have been better just leaving in savings account/ premium bonds but I guess couldn't predict whats happened over the last year.