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Under certain situations, a Great Ball is more effective than an Ultra Ball. In particular, Pokémon with high catch rates, no status, and above ½ HP may guarantee capture with a Great Ball but fail to do so with an Ultra Ball.
The effect of reducing HP diminishes with lower catch rates, and the effect of inflicting status diminishes with higher catch rates. Regardless of catch rate, reducing the HP does not increase chances of capture at less than ½ HP with Great Balls, and ⅓ HP with other balls.
Inflicting a status ailment gives the player a flat additional chance (from 4.6% to 16.6%) to catch the Pokémon regardless of all other factors, making certain Pokémon easier to catch in Generation I than they are in later generations: A sleeping Mewtwo with full HP may, on average, be caught in about six Ultra Balls in Generation I, but from Generation II onwards exactly the same circumstances would require an average of about 64 Ultra Balls— over ten times as many.
Due to the nature of the algorithm, Ultra Balls will only perform better than Great Balls on Pokémon whose capture rates are above 55 and below 200 in Generation I. Ultra Balls increase the overall chance of capture by as much as 20% in comparison to Great Balls for Pokémon near the center of that range.
Assuming the current HP of target Pokémon is less than or equal to ½ HP for Great Balls and ⅓ HP for Ultra Balls, Ultra Balls increase the overall chance to capture by 10% or more in comparison to Great Balls for the following Pokémon:
- Metapod, Kakuna, Pidgeotto, Raticate, Fearow, Arbok, Sandslash, Nidorina, Nidorino, Clefairy, Jigglypuff, Golbat, Gloom, Persian, Poliwhirl, Kadabra, Machoke, Weepinbell, Graveler, Haunter, Exeggcute, and Rhyhorn.