Maths Olympiad Prep

Track / Stage 5 / 389 of 400 #989 of 1964

Problem 989

AIME late
Algebra Difficulty 6.0 Find the answer

1817. The distribution of features XX and YY is presented in the following correlation table:

01020304050607080mxm_{x}
-21214
-113318
0244212
1155112
23

Find the correlation ratios ηy/x\eta_{y / x} and ηx/y\eta_{x / y} and compare them with the corresponding linear correlation coefficient.

A number or a short expression. Spacing, $ signs and \frac vs / are all fine.

Official solution

Solution. The calculation of correlation relationships will be carried out according to the following scheme (table on p. 410):

Xˉ=ΣxmxxΣxmx=842=0.190;Xˉ2=Σxmxx2xmx==6042=1.424;σX2=X2(Xˉ)2=1.388; \begin{aligned} & \bar{X}=\frac{\Sigma_{x} m_{x} x}{\Sigma_{x} m_{x}}=\frac{8}{42}=0.190 ; \bar{X}^{2}=\frac{\Sigma_{x} m_{x} x^{2}}{\sum_{x} m_{x}}= \\ & =\frac{60}{42}=1.424 ; \sigma_{X}^{2}=\overline{X^{2}}-(\bar{X})^{2}=1.388 ; \end{aligned}

Vˉ=Σmyvymy=042=0;V2=Σmyv2Σmy=30242=7.190;σV2=7.190;(Vx)2=x1mx(yΣmxyv)2Σxmx=042=0;σ2(Vx)=(Vˉx)2(Vˉ)2=00=0;(Xˉ)y)2=y1mˉy(xmxy)x)2ΣyΣmy=52.54242=1.251;σ2(Xv)=(Xˉv)2(Xˉ)2=1.215. \begin{gathered} \bar{V}=\frac{\Sigma m_{y} v}{\sum_{y} m_{y}}=\frac{0}{42}=0 ; \overline{V^{2}}=\frac{\Sigma m_{y} v^{2}}{\Sigma m_{y}}=\frac{302}{42}=7.190 ; \sigma_{V}^{2}=7.190 ; \\ \overline{\left(\overline{V_{x}}\right)^{2}}=\frac{\sum_{x} \frac{1}{m_{x}}\left(\sum_{y}^{\Sigma} m_{x y} v\right)^{2}}{\Sigma_{x} m_{x}}=\frac{0}{42}=0 ; \sigma^{2}\left(\overline{V_{x}}\right)=\overline{\left(\bar{V}_{x}\right)^{2}}- \\ -(\bar{V})^{2}=0-0=0 ; \\ \overline{\left.(\bar{X})_{y}\right)^{2}}=\frac{\left.\sum_{y} \frac{1}{\bar{m}_{y}}\left(\sum_{x} m_{x y}\right)^{x}\right)^{2}}{\Sigma_{y}^{\Sigma m_{y}}}=\frac{52.542}{42}=1.251 ; \sigma^{2}\left(\overline{X_{v}}\right)=\overline{\left(\overline{\bar{X}_{v}}\right)^{2}}- \\ -(\bar{X})^{2}=1.215 . \end{gathered}

!

From this, considering property 5, ηg/x=ηo/x=σ2(Vˉx)σV2=07.190=0;ηx/g=ηx/D=σ2(Xˉv)σX2=1.2151.388=0.875=\eta_{g / x}=\eta_{o / x}=\sqrt{\frac{\sigma^{2}\left(\bar{V}_{\mathrm{x}}\right)}{\sigma_{V}^{2}}}=\sqrt{\frac{0}{7.190}}=0 ; \eta_{x / g}=\eta_{x / D}=\sqrt{\frac{\sigma^{2}\left(\bar{X}_{v}\right)}{\sigma_{X}^{2}}}=\sqrt{\frac{1.215}{1.388}}=\sqrt{0.875}= =0.935=0.935. Obviously, ρy/x=ρx/y=0\rho_{y / x}=\rho_{x / y}=0 and r(X,Y)=0r(X, Y)=0.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.