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20.4 Large or small constants in a distribution

When numbers with magnitude less than 0.1 or greater than 10 are used as arguments to a distribution, it indicates that some parameter is not scaled to unit value, so a warning is thrown. See https://mc-stan.org/docs/stan-users-guide/standardizing-predictors-and-outputs.html for a discussion of scaling parameters.

For example, consider the following program.

parameters {
  real x;
  real y;
}
model {
  x ~ normal(-100, 100);
  y ~ normal(0, 1);
}

The constants -100 and 100 suggest that x is not unit scaled.

Pedantic mode produces the following warning.

Warning at 'constants-warn.stan', line 6, column 14 to column 17:
  Argument -100 suggests there may be parameters that are not unit scale;
  consider rescaling with a multiplier (see manual section 22.12).
Warning at 'constants-warn.stan', line 6, column 19 to column 22:
  Argument 100 suggests there may be parameters that are not unit scale;
  consider rescaling with a multiplier (see manual section 22.12).