Paper 4, Section I, J

Statistical Modelling
Part II, 2018

A scientist is studying the effects of a drug on the weight of mice. Forty mice are divided into two groups, control and treatment. The mice in the treatment group are given the drug, and those in the control group are given water instead. The mice are kept in 8 different cages. The weight of each mouse is monitored for 10 days, and the results of the experiment are recorded in the data frame Weight.data. Consider the following RR code and its output.

head (Weight.data)

Time Group Cage Mouse Weight

11 Control 1 1 24.7757824.77578

222 \quad 2 Control 1 1 24.6876624.68766

333 \quad 3 Control 1124.790081 \quad 124.79008

44 Control 1124.770051 \quad 124.77005

555 \quad 5 Control 1 1 24.6509224.65092

666 \quad 6 Control 1124.384361 \quad 124.38436

>mod1=lm>\bmod 1=\operatorname{lm} (Weight \sim Time*Group ++ Cage, data=Weight. data)

>summary(mod1)>\operatorname{summary}(\bmod 1)

Call:

lm\operatorname{lm} (formula == Weight  ~Time * Group ++ Cage, data == Weight. data)

Residuals:

Min 1Q1 Q Median 3Q3 Q Max

1.369030.335270.017190.388071.24368-1.36903-0.33527-0.01719 \quad 0.38807 \quad 1.24368

Coefficients:

Estimate Std. Error t value Pr(>t)\operatorname{Pr}(>|t|)

 Time 0.0060230.0126160.4770.63334\begin{array}{lllll}\text { Time } & -0.006023 & 0.012616 & -0.477 & 0.63334\end{array}

GroupTreatment 0.3218370.1219932.6380.00867\quad 0.321837 \quad 0.121993 \quad 2.638 \quad 0.00867 *

Cage2 0.4002280.0958754.1743.68e05\quad-0.400228 \quad 0.095875-4.1743 .68 \mathrm{e}-05 * * *

 Cage3 0.2869410.1024942.8000.00537\begin{array}{lllll}\text { Cage3 } & 0.286941 & 0.102494 & 2.800 & 0.00537 *\end{array}

 Cage4 0.0075350.0958750.0790.93740\begin{array}{lllll}\text { Cage4 } & 0.007535 & 0.095875 & 0.079 & 0.93740\end{array}

 Cage6 0.1247670.1255300.9940.32087\begin{array}{rrrrr}\text { Cage6 } & 0.124767 & 0.125530 & 0.994 & 0.32087\end{array}

 Cage8 0.2951680.1255302.3510.01920 Time:GroupTreatment 0.1735150.0178429.725<2e16\begin{array}{lllll}\text { Cage8 } & -0.295168 & 0.125530 & -2.351 & 0.01920 * \\ \text { Time:GroupTreatment } & -0.173515 & 0.017842 & -9.725 & <2 e-16 * * *\end{array}

Time: GroupTreatment 0.1735150.0178429.725<2e16-0.173515 \quad 0.017842-9.725<2 \mathrm{e}-16 * * *

Signif. codes: 0 '' 0.0010.001 '' 0.010.01 '' 0.050.05 '., 0.10.1 ', 1

Residual standard error: 0.51250.5125 on 391 degrees of freedom

Multiple R-squared: 0.55910.5591, Adjusted R-squared: 0.550.55

F-statistic: 61.9761.97 on 8 and 391 DF, p-value: <2.2e16<2.2 \mathrm{e}-16

Which parameters describe the rate of weight loss with time in each group? According to the R\mathrm{R} output, is there a statistically significant weight loss with time in the control group?

Three diagnostic plots were generated using the following RR code.

Weight.data$Time[mouse1]

Weight.data$Time[mouse2]

Based on these plots, should you trust the significance tests shown in the output of the command summary (mod1)? Explain.