The situation in which a random variable, Y, has expected value g(x1, x2,…, xm), where x1, x2,…, xm are m explanatory variables (see regression) and g is an unknown function. With m=1 and n observations, y1, y2,…, yn, estimation may proceed using a scatterplot smoother such as loess, or by using kernel regression. In the latter case, to estimate the function g in the regression equation
a commonly used procedure employs the Nadaraya-Watson estimator given by
where K is the chosen kernel (see kernel method).