Here are some general examples of the influence of market condition on the VA rule:
When the market is an a trading range – coming into balance – you often get 1 day up 1 day down sort of action. During such a noisy environment, in general the odds of the prior day VA getting filled would be higher than average. See Chart 1
When this happens the market is effectively carving out a higher timeframe value area, which will often be reflected by a 60 / 120min low ADX. See Chart 2 (same 4 days in Bund as Chart 1)
(This biref Steidlmayer article is worth reading.)
In a trending market, an open out of value in the direction of the trend (i.e. above the VA in an up trend) that then tests back into the VA, is generally less likely to fill. Gaps against the trend – for instance an open down below the prior VA after a high-to-low day in an up trend – are more likely to fill than average (this scenario would be analogous to a pinball buy). See chart 3
Dalton: “The direction of the current longer-term auction has an obvious influence on the momentum, or strength behind the value area penetrartion. When price auctions up into value during a buying trend, for example, the chances of continuation are much better than if the market were in a downward trend."
There are also some conceptual issues to be considered when assessing the value area of a particular day:
1. Value is created by rotations. Although a Value Area can always be calculated, those days which auction continuously in one direction are indicative of a market that is searching for a fair price. In other words those days that exhibit little price rotation (i.e. trend days) do not produce a true Value Area.
2. Dalton: “Narrow value is a sign of poor trade facilitation and lower volume. Because volume slows price, narrow value areas are more easily traversed than wider, high volume areas. Therefore, the Value Area Rule carries a higher probability when price enters a narrow value area.”
These are just some general points. Others may disagree with them and I do not in any way wish to present myself as some sort of expert. But, I believe these examples go some way towards explaining why, when assessed without taking into account market conditions or market logic, the VA rule is closer to a 50% tendency.