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The book gave examples of raising partner's major by counting tricks, which mainly comes from honours and shortages rather than trump lengths. For example, it advised against raising 1♥-(1♠) with ♠xx ♥xxxxx ♦Kx and ♣xxxx because the strength was not above the opener's assumed average, but raise with ♠Kxx ♥Qx ♦KQxx ♣Axx because it contained 1 trick more than the assumed strength, or ♠ ♥xxxx ♦Kxxx ♣Qxxxx where the ♠ void and 4 small trumps together were counted as 3 tricks.
However, in modern bidding, the hand with 5 small trumps support is a good preemptive raise from 1 to 4 to shut out the opponents, even if it doesn't necessarily mean taking more tricks.
When and why did the bidding style changed to, in competitive bidding, bid according to the length instead of the strength?