function betahat = regression(x_regr,z_regr,beta,model_str);
[betahat,resid,J] = nlinfit(x_regr,z_regr,model_str,beta);
betaci = nlparci(betahat,resid,J);
[yhat,delta] = nlpredci(model_str,x_regr,betahat,resid,J);
opd = [z_regr yhat delta];