Part science, part magic - a decent strategy is essential to the business of winning races. Or, indeed, losing them. The basic variables of the equation are simple enough: fuel load and tyre wear. But from then on, it gets vastly more complicated.
The black art of race strategy is constantly evolving, but goes through particularly marked transitions when major rule changes are introduced. Shortly after the reintroduction of refuelling stops in 1994, the teams' race strategists worked out that at some circuits benefit could be gained from making two or three stops, rather than just one.
This was because the car could run substantially quicker on a lower fuel load (with less weight to carry around) and using the grippier, but less durable, soft tyre compounds. The difference in performance was such that it could be sufficient to offset the effect of the 30 or so seconds lost making a pit stop.
That led to teams carefully working out just where in the order their driver would re-emerge after a stop. This allowed a car being baulked by a slower but hard to overtake runner to pit early, return to clear track and then put in faster laps that would ensure emerging ahead once the slower car made its stop - overtaking in the pit lane as it has become known. This called on rigid pit stop timetables to be abandoned and replaced by a looser system of pit stop windows, with a number of laps on which a car can make its stop to gain best strategic advantage.
The move to a single tyre supplier in 2007 forced teams to once again re-evaluate their race strategies, in light of all their rivals running on the same rubber and the requirement for all drivers to use both the supplied specifications of dry tyres during a race. That was followed by the ban on refuelling for the 2010 season, obliging teams to once again reconsider how they plan their race.
Regardless of rule changes, there are certain factors that must always be considered. Data such as weather forecasts, the likelihood of overtaking at a particular track, the length of the pit lane and even the chances of an accident likely to require the use of the safety car all come into play when deciding strategy. And, of course, one of the largest ingredients remains, as always, luck.