= Psychometric function with guessing and lapses
{title2=$p(x)=\gamma+(1-\gamma-\lambda)F(x)$}
An empirical <psychometric function> whose core $F$ ranges from zero to one, lower asymptote is $\gamma$, and upper asymptote is $1-\lambda$. If $F(\alpha)=1/2$, then $p(\alpha)=\gamma+(1-\gamma-\lambda)/2$. A location parameter is consequently not a task-independent threshold. For a two-choice observer randomly guessing on fraction $r$ of otherwise perfect easy trials, the upper error asymptote is $r/2$, not $r$.
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