private double evaluateInternal(ValueVector x, ValueVector y, int size)
throws EvaluationException {
// error handling is as if the x is fully evaluated before y
ErrorEval firstXerr = null;
ErrorEval firstYerr = null;
boolean accumlatedSome = false;
// first pass: read in data, compute xbar and ybar
double sumx = 0.0, sumy = 0.0;
for (int i = 0; i < size; i++) {