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 This is good: it gets our model out of the gate. If investors were not broadly homogeneous, our statistics would not work. “What is the average height of all things” is not a meaningful calculation.
-But there is a second order sense in which the earlier and later trades are related: the later participants know about the earlier trade and its price — and its position in a trend from the trade before that —and they will use l this abstract information to form their bid and ask.
+But there is a second order sense in which the earlier and later trades are related: the later participants know about the earlier trade and its price — and its position in a trend from the trade before that — and they will use l this abstract information to form their bid and ask. This interconnectedness of transactions means they are not entirely independent, but the immediate transaction history is pretty chaotic and, as traders say, “noisy”— in the immediate term, the dissimilarities between trader motivations are most pronounced: it is only over a large aggregate that they cancel themselves out — that is to say a “signal” only emerges from the noise over time. If all traders are using market information, this immediate interdependence looks like independence. So a stochastic model works fairly well. In the same way impurities and intervening causal interference means the the conditions are never quite right to fully satisfy Newton ’s model, and the world never quite obeys it, but does so only roughly, so it is with statistical techniques in the market. The occasional intervention of idiosyncratic behaviour is basically noise.
 buy or sell. In the modern days of computerised trading everything is very clean, tidy observable, unitary and discrete.