It is common in science and engineering to make a graph of experimental data for the purpose of discovering the slope. Often we need to report the slope with a confidence interval. For example, we may need to report the value of the slope is 1.23 ± 0.34. The diagram shows an example of this calculation. Click here to get sample workbook.

The confidence interval is computed using _{} where *t *is the
critical value of the *t* statistic and *S _{b}* is the
standard error in the slope.

The *t* value is computed as **=TINV(a,df)**.
If we were to use a table to look up *t* at the 95% confidence level, we
would look the t value corresponding to a =½(1-95%) = 0.025. However, since in
the Excel function TINV, a is the probability associated with the two-tailed Student's
t-distribution, we use a value of

1 - 95% = 0.05. The degree of freedom is the number of observations less 2.

The *S _{b}* value is readily obtained
from the LINEST function. If we do not need all the LINEST output we may use

Note: the plus-minus symbol (±) is produced with the keystrokes ALT+0177.