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and here is how to catch Pokémon in the Generation III-IV

Modified catch rate
The modified catch rate, a, is calculated in Generation III and Generation IV as follows:

• HPmax is the number of hit points the Pokémon has at full health,
  
• HPcurrent is the number of hit points the Pokémon has at the moment,
  
• rate is the catch rate of the Pokémon (which may be modified due to use of apricorn balls or actions in the Safari Zone),
  
• bonusball is the multiplier for the Poké Ball used
  
• bonusstatus is the multiplier for any status condition the Pokémon has (2 for sleep and freeze, 1.5 for paralyze, poison, or burn, and 1 otherwise).

If a Pokémon could have 0 HP, the maximum value for a would be rate × bonusball × bonusstatus. For a Pokémon with full health and no status condition, and with a neutral ball used, the minimum value for a would be rate / 3.

The formula is slightly different when applied to the Apricorn balls in HeartGold and SoulSilver. The modifiers for these balls are applied directly to the Pokémon's catch rate, rather than in the formula. In this case, bonusball is always 1, and the catch rate cannot go higher than 255. This means that for Pokémon whose catch rate is already 255, such as Rattata, the Apricorn balls do not make the capture any more likely than a regular Poké Ball.

If you need more help: http://bulbapedia.bulbagarden.net/wiki/Catch_rate