Hi,
Can you help me out?
I've data as follows:
X Y
8 5.93
10 5.87
12 5.84
I want to extapolate the graph and find out the value where this graph cuts the Y-axis i.e. values for 0,2,4 & 6.
Put the data into Excel (or some other spread sheet) and plot it as a scatter plot. Select the line and right click and select add trend line, select linear for the type of trend.
Now right click on the trend line and select format trend line, click on display equation, also setup forcast to forcast backwards by 8 units.
Now read all you need off of the plot.
The attachment shows what you should have
RonL
See here for the equations.
Summarising these:
Put
$\displaystyle s_{x,y}=sum(x_i y_i)$
$\displaystyle s_{x}=sum(x_i)$
$\displaystyle s_{y}=sum(y_i)$
$\displaystyle s_{x,x}=sum(x_i^2)$
$\displaystyle s_{y,y}=sum(y_i^2)$
Then:
$\displaystyle m=\frac{n s_{x,y}-s_x s_y}{n s_{x,x}-s_x^2}$,
where $\displaystyle n$ is the number of data points.
and:
$\displaystyle b=\frac{s_y-m s_x}{n}$.
Then the regression line is $\displaystyle y=m x+b$ and $\displaystyle y=b$ when $\displaystyle x=0$.
RonL