Bowling strategy (cricket)
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In cricket, at the start of an innings the pace bowlers on the bowling team usually bowl first. Usually the pace bowlers will bowl for 10 overs in a One Day International or about two hours in a Test match. Pace bowlers traditionally bowl first since the cricket ball is initially hard and smooth, and hence moves quickly through the air, bounces quickly off the pitch and does not spin much. Generally the fast bowler's job is to get as many early wickets (batsmen out) as possible and make inroads into the opposing batting line-up.
The main objective of the bowler is to take wickets. Usually the batting order is such that the best players bat first. As the bowling team gets more wickets, the lesser skilled batsmen come to face the bowling. If the bowling team can take quick wickets, it can reduce the confidence of the batting team and can help keep their score low.
The second objective of the bowler is to prevent the batsmen scoring runs. A bowler's effectiveness at keeping the batsmen from scoring runs is measured by the bowler's economy rate. Economy rate is the average number of runs scored per over bowled. If the bowler succeeds in keeping the economy rate below 3 an over, for say 86 of his overs, this can create pressure on the batsman to score runs. This sometimes induces the batsman to play more risky shots, which may result in wickets being taken through catches and stumpings.
The team is required to bowl a set number of overs within a time frame. The number of overs bowled per hour is known as the over rate. Failure to maintain the over rate above the stipulated time would invite a monetary penalty imposed by the umpires.
Line & length 
The bowler must bowl a tight line and length. The best line is dependent upon the exact location of the fielders, but the most commonly strived for line generally lies between middle stump (with a leg-side field) to 6 inches (150 mm) outside off-stump (with an off-side field).
A ball of the ideal length will bounce up to around the batsman's thigh level. At this height, it is difficult for the batsman to freely play his stroke. The ideal length is such that the batsman is unsure whether to move his weight onto the front foot or the back foot to play his stroke. But bowlers should also vary the length, i.e. vary the bounce so that each ball is not predictable. A bouncer (short pitched ball which rises above the batsmans neck) can unsettle the batsman.
One day internationals 
After bowling the first few overs of the match, fast bowlers usually return to bowl the last 10 overs (40th-50th). There is no fixed rule as to when a bowler can bowl, as long as he bowls a maximum 10 overs. He is not allowed to bowl 2 or more consecutive overs.
Usually the best fast bowlers bowl the last few overs. This tactic is employed to try to prevent the batsmen 'going after' the spinners and scoring many runs. Usually the batsmen will play more aggressively during the final few overs, since they are less concerned with getting out, and more concerned with maximising the number of runs scored.
After the 15 overs are up, the game tempo usually slows down a bit. There aren't too many boundaries (4s and 6s) hit. After the 35th over, the batsmen tend to be more aggressive to score quickly, so during this period, the most experienced bowlers bowl. The last 15 overs are known as the slog or death overs. As the batsmen are looking to hit as many big hits as possible, bowlers get hit around and their economy rate worsens. However this is also the most productive part of the game for the bowlers as wickets tumble here.
If the team bowling in the second innings have a good total to defend, they may try to slow the run rate rather than bowling aggressively to take wickets. This can cause the batting team to begin taking unnecessary risks and result in wickets falling. Taking wickets is crucial to win the match as the more wickets the opposition have in hand (not-out batsmen) the greater the chances of the batsmen scoring runs rapidly. If the bowlers are defending less than say 200 runs, wicket taking is imperative to win the match. Bowlers must remember that there is a strong correlation between the batting side losing wickets and a slowing of their run rate. Two new batsmen at the crease always have to consolidate and more often than not the run rate will slow.
Test cricket 
In Test cricket, the overs are unlimited. As a consequence, a bowler's economy rate is lower than that of a One Day International match. For a bowler, getting wickets is the main task. The more wickets a bowler takes, the less a chance the batsmen can score a big total. The margin for error in bowling wide balls in Test matches is much higher, so the bowler need not be as accurate as in a One Day International match. He may also bowl 2 bouncers per batsman per over.
Changing length 
A useful variation in bowling is changing length. It is used to make batsman play back foot or front foot. You can trick the batsman by bowling 3-4 short balls and then bowl a fuller delivery. Else, you can bowl 3-4 fuller delivery and then bowl a bouncer and get batsman caught at leg gully or short fine leg.
Changing Pace 
A common variation in modern bowling is to vary the pace of the ball or the "delivery". Unlike variations in line and length, the variation in pace is often used to trick the batsman into playing the shot earlier or later, as he is anticipating the ball to come towards him at a certain pace. This tactic is especially effective on pitches that are unhelpful for the bowlers. Pace variations are a more commonly(though not exclusively) tactic practiced by spin bowlers. Spinning deliveries will usually turn more if they are slower. Fast and medium pace bowlers may bowl slower and impart a bit of a "break" to the ball and bowl off "cutters" and leg "cutters" to further the variation-thereby creating a faster form of spin bowling, which angles inward or outward rather than spinning. Slowing the pace of the ball may also be used as a tactic to prevent the batsman from simply using the pace of the ball to guide it away for easy runs, especially in case of a batsman friendly track and a fast outfield.