Transaction volume (notional dollar-volume) is not directly measurable — that’s true. However, tick-volume (a count of price changes per time period) is measurable, and is readily available in real time in almost every trading platform.
And multiple studies have demonstrated that transaction volume and tick-volume move in tandem. That is, the pattern of rising and falling volume, and the occurrence of volume spikes, that we see in a tick-volume chart — is a reliable proxy for the transaction volume, which we can’t see.
The only significant difference between tick-volume and transaction volume is that tick-volume [I]includes numbers[/I] (an actual count of the price changes in each time period), whereas actual transaction volume (dollar-volume) [I]is not retrievable numerically[/I] by ordinary retail traders.
There is a very large thread — 301 Moved Permanently — here on this forum which deals with the application of volume analysis to the trading of price patterns and candlestick patterns, such as the doji example you referred to.
Are you familiar with that thread? If not, you should check it out.