2PL probability function The probability function in the 3PL model.
| 1 | probability.2pl(z, a = z$a, b = z$b, theta, d = -a * b)
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| z | Optional. A list with the parameters a and b specified by keys. | 
| a | The discrimination parameter | 
| b | The difficulty parameter. (Optional if d is specified) | 
| theta | The subject's latent trait. | 
| d | Optional. Overrides the b parameter, it is equal to -a*b. Used in some functions. | 
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