# Activity 09/09/2020 ## STAT 757 -- Section 1001
Instructor: Colin Grudzien
## Instructions: We will work through the following series of activities as a group and hold small group work and discussions in Zoom Breakout Rooms. Follow the instructions in each sub-section when the instructor assigns you to a breakout room. ## Activities: ### Activity 1: sample means #### Question 1: Recall our assumption that $\mathbb{E}\left[\epsilon_i\right]=0$ for every $i$. We will define the sample-based mean of the variations as \begin{align} \overline{\epsilon} = \frac{1}{n}\sum_{i=1}^n \epsilon_i. \end{align} Discuss with peers: knowing that the mean of the random variable $\epsilon_i$ is zero for every $i$, does this imply that the sample-based mean $\overline{\epsilon}=0$? Explain why or why not. Then use the above assumptions to find the expected value of $\overline{\epsilon}$. ### Debrief: We will discuss the solution to activity 1 as a class. ### Activity 2: unbiased estimators #### Question 1: It was mentioned that $\hat{\beta}_0$ and $\hat{\beta}_1$ are unbiased estimates of $\beta_0$ and $\beta_1$. Using the definitions of the point-estimates for the parameters $\beta_0$ and $\beta_1$ by ordinary least squares: \begin{align} \hat{\beta}_1 &= \frac{\sum_{i=1}^n\left(X_i - \overline{X} \right)\left(Y_i - \overline{Y}\right)}{\sum_{i=1}^n\left(X_i - \overline{X} \right)^2}, \\ \hat{\beta}_0 &= \frac{1}{n} \left(\sum_{i=1}^n Y_i - \hat{\beta}_1 X_i \right), \end{align} let us now prove that this is the case. ### Debrief: We will discuss the solution to activity 2 as a class. ### Activity 3: Correlation and covariance #### Question 1: Using the definition of the estimated parameter value $\hat{\beta}_1$ by least squares above, can you derive how this coefficient is related to \begin{align} r = \frac{S_{XY}^2}{S_X S_Y}, \end{align} the sample-based correlation between $X$ and $Y$? Recall the definitions of the sample-based covariance and standard deviations. #### Question 2: Following the previous question, recall the following facts: 1. $S_{XY}^2$ is an unbiased estimator of $\mathrm{cov}(X,Y) = \sigma_{XY}^2$ 2. By definition, the covariance of two variables $X$ and $Y$ is $\sigma_{XY}^2 = 0$ if and only if the correlation between these variables $\rho_{XY}=0$ is zero. Let's suppose that the $X_i$ are treated as deterministic constants and the variables $X$ and $Y$ are uncorrelated such that $\rho_{XY}=0$. Using the definition of the point estimate $\hat{\beta}_1$ and its relationship to the correlation between $X$ and $Y$, what is the implication for $\beta_1$ and the regression function when $X$ and $Y$ are uncorrelated? ### Debrief: We will discuss the solution to activity 3 as a class. ### Activity 4: Gaussian error distibutions (take home) #### Question 1: We stated in the lectures that it can be demonstrated that linear combinations of Gaussian distributed random variables are also Gaussian. If the variation in the signal is drawn iid $\epsilon_i \sim N(0,\sigma^2)$, which of the following can we deduce are also Gaussian distributed? 1. $X_i$; 2. $Y_i$; 3. $\hat{\beta}_0, \hat{\beta}_1$; 4. $\hat{Y}_i$; 5. $\hat{\epsilon}_i$.