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A caveat for bivariate data
Predicting using PREDY
In the
SYMB
view (see right) the equation is given to so many decimal
places that it doesn’t fit onto the screen. The simplest way to see the
entire equation is to position the highlight on the equation and press the
key.
When you do you will obtain the view seen right which gives the
equation of the line of best fit as
y
ˆ
=
0.8199
x
+
1.1662
. With a
correlation coefficient of 0.7829 (from the summary stats seen
previously) this would probably not be regarded as reliable.
On the screen above the equation is given to 4 decimal place accuracy.
This is accomplished by changing to the
MODES
screen (above the
HOME
key) and specifying
Fixed 4
as the
Number Format
. If you
don’t do this then the alternative is to use the arrow keys to scroll the
equation left or right if it extends off the screen.
Calculator Tip
instead of
. Always use
One of the most common mistakes people make is to press
when you want to display the
entire equation. An explanation is given below.
instead of
.
Although pressing
It is very important that you display the equation in the
SYMB
view by pressing
displays the equation in a form which can be scrolled through, the problem with this
method is that the calculator thinks that by editing it you are choosing to over-ride its choice of equation with
your own. As long as you press
there is no problem but if you press
ENTER
it changes the
FitType
from whatever you had previously chosen to
User Defined
instead.
If you are not aware that this has happened then it can have very serious consequences. For example, if you
were now to change the data and then re-plot, the fit equation will not be re-calculated and will become
inaccurate. Any predictions made from it would be incorrect.
We can make predictions from our line of best fit in two places - the
HOME
view and the
PLOT
view.
In the
HOME
view we use the functions
PREDY
and
PREDX
from the
Stat-Two section of the
MATH
menu. The functions
PREDX
and
PREDY
use whatever was the last line of best fit calculated. It is up to you to
ensure that the one you want used was the one last graphed. If I want to
predict a y value for x = 3, then I simply type
PREDY(3)
into the
HOME
view as shown right. Many people choose to simply type ‘
PREDY
’
using the
ALPHA
button instead of going through the
MATH
menu.
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