# Meaning of θj in equation for partial derivative of MSE

The equation to find the partial derivative of a cost function with respect to a parameter θj is given in the book Hands-On Machine Learning with Scikit-Learn, Keras, and TensorFlow:

$$\frac{\partial}{\partial \theta_j}\operatorname{MSE}({\boldsymbol{\theta}}) = \frac{2}{m} \sum_{i=1}^m \left(\boldsymbol{\theta}^\intercal \boldsymbol{x}^{(i)} - y^{(i)}\right)x_j^{(i)}$$

• m = number of instances in the dataset
• x = input vector for the prediction
• y = label for the input vector

Is $$\theta_j$$ in the equation a unique combination of bias and weights which is reflected in the vector $$\boldsymbol{\theta}$$ in the equation? Or is it a single parameter inside a vector?

In response to question and also following comment:

Sorry, but could I ask a small follow-up question? If θj = 5 then should the jth element in θ also be 5? E.g - If on the left hand side θj= θ3 = 5 then should the θ on the right-hand-side of the equation be [θ0, θ1, θ2, 5, θ4, θ5, θ6]?

Which is at:

Meaning of a varaible for calculating the partial derviative of MSE cost function

Take care - you cannot apply analagous reasoning as you have in your comment above as for the question you asked previously, concerning $$\mathbf{x}^{(i)}$$.

Concerning:

Is $$\theta_j$$ in the equation a unique combination of bias and weights which is reflected in the vector $$\boldsymbol{\theta}$$ in the equation? Or is it a single parameter inside a vector?

The answer is the latter; $$\frac{\partial \operatorname{MSE}(\boldsymbol{\theta})}{\partial \theta_j}$$ is the partial derivative of the MSE with respect to a single, $$j$$-th element of the parameter vector $$\boldsymbol{\theta}$$. Whereas $$\boldsymbol{\theta}$$ on the RHS is a vector.

If that continues to remain unclear, and you are unable to see how equation (4.5) and equation (4.6) are connected in Geron's book, prompt and I will be happy to unpack the linear algebra.

To the best of my knowledge, I am unable to detect the presence of a typo.

Yes, and Yes. Let me explain.

In mathematics this is called a gradient, i.e. a vector. This means that you take a partial derivative over each parameter of a scalar function, which forms a vector. So, each of the elements is a partial derivative of the function over one parameter, but together they are a gradient.

Since it's a vector, it must be pointing somewhere. It does point in the direction of the greatest sensitivity to the change of the parameters of the function.