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I am looking through some implementation of linear regression, I found it is not calculating parameter directly using formula like below,

enter image description here

but calculate Pearson product-moment correlation coefficient , then estimate parameter using Pearson's coefficient.

Here is an example, I think result is the same, but just confused why not using formula I posted above to calculate slope and intercept directly? What is the benefit if calculating Pearson's coefficient first?

http://code.activestate.com/recipes/578914-simple-linear-regression-with-pure-python/

def fit(X, Y):

    def mean(Xs):
        return sum(Xs) / len(Xs)
    m_X = mean(X)
    m_Y = mean(Y)

    def std(Xs, m):
        normalizer = len(Xs) - 1
        return math.sqrt(sum((pow(x - m, 2) for x in Xs)) / normalizer)
    # assert np.round(Series(X).std(), 6) == np.round(std(X, m_X), 6)

    def pearson_r(Xs, Ys):

        sum_xy = 0
        sum_sq_v_x = 0
        sum_sq_v_y = 0

        for (x, y) in zip(Xs, Ys):
            var_x = x - m_X
            var_y = y - m_Y
            sum_xy += var_x * var_y
            sum_sq_v_x += pow(var_x, 2)
            sum_sq_v_y += pow(var_y, 2)
        return sum_xy / math.sqrt(sum_sq_v_x * sum_sq_v_y)
    # assert np.round(Series(X).corr(Series(Y)), 6) == np.round(pearson_r(X, Y), 6)

    r = pearson_r(X, Y)

    b = r * (std(Y, m_Y) / std(X, m_X))
    A = m_Y - b * m_X

    def line(x):
        return b * x + A
    return line
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What is the benefit of computing the projection matrix $H=X(X'X)^{-1}X'$ and then take $H\beta=\hat{Y}$ instead of calculating the intercept and the slope using the OLS results?

In the simple linear model $y=\beta_0 + \beta_1x+\epsilon$ where $\epsilon \sim \mathcal{N}(0, \sigma^2) $. It doesn't matter whether you compute estimators using OLS, Pearson, MLE or Projection matrix. However, in multiple regression models, you can not use the Pearson correlation coefficient because you have more than two variables to correlate. If the noise term is not normal, then the Pearson coefficient (and the OLS) may by inappropriate. So, using $\hat{\beta}_1=r\frac{\sigma_{Y}}{\sigma_{X}}$ is approapriate and equivalent to the OLS result for the simple model with the aforementioned assumptions, which is only a special case of a linear parametric model.

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  • $\begingroup$ Nice catch AE, vote up. What do you mean "compute estimators"? I could be wrong, but I think estimator is some algorithm estimate parameter (slope and interception), it should be like least square error method, Pearson is used to check how good linear fit for independent or dependent variable? But it seems you mean Pearson is estimator other than least square error? Did I mis-understand anything from your side? $\endgroup$
    – Lin Ma
    Sep 13 '16 at 7:35
  • $\begingroup$ And OLS means Optimize Least Square? Or something else? $\endgroup$
    – Lin Ma
    Sep 13 '16 at 7:36
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    $\begingroup$ In the terminology of classical stats, $\beta_0, \beta_1$ are unknown parameters that we estimating using the Ordinary Least Squares (OLS) algorithm. As you can see here there is an one-to-one functional relationship between the OLS estimator of $\beta_1$ and Person's $r$. Hence, knowing $r$ and the sample variances you can find the estimated linear model, and vice versa. $\endgroup$
    – V. Vancak
    Sep 13 '16 at 16:54
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    $\begingroup$ You are welcome. So the short answer is that there is no benefit, because in the settings of a simple linear model, the information conveyed by the OLS estimators and the Pearson $r$ is basically the same. $\endgroup$
    – V. Vancak
    Sep 14 '16 at 9:27
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    $\begingroup$ There are Generalized Least Squares (GLS), their special case - (Weighted) WLS, and (Feasible) FGLS. As such, the OLS are the simplest least square estimators. $\endgroup$
    – V. Vancak
    Sep 15 '16 at 14:49

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