Error

Error Error is the difference between a measured or

calculated (reported) value and the true value. Engineers recognize that errors are present in

their professional lives and must be able to:

1. Identify types of errors

2. Numerically express the magnitude of errors

3. Recognize the confidence that may be placed in a printed number

Simple Error Analysis Suppose a rod of unknown length is

measured with a standard meter stick. Spend 5 minutes as a team completing

this exercise: What can be said about the length of the

rod is reported as:

7 to 8 cm

7.5 to 7.6 cm

7.57 to 7.59 cm?

Systematic Errors Systematic Errors - errors that can be

attributed to some regular outside occurrence.

Engineers must be aware of the presence of systematic errors and eliminate those possible and try to quantify and correct for those remaining.

Systematic Errors The error associated with systematic

errors can be corrected if the source and magnitude are known.

Repeating measurements will not eliminate or reduce systematic errors.

Random Errors Accidental (Random) Errors - errors

that occur in a random nature.

The presence of accidental errors is evident by the scatter in measured data.

Random Errors It is impossible to predict the magnitude

and sign of the accidental error present in any one measurement.

Repeating measurements and averaging the results will reduce the random error in the average.

What type of error is it?The produce scale at the grocery store has water on

it. (The water runs off the produce)

The timekeeper sneezes at the moment the runners cross the finish line.

Gasoline sloshed from your tank prior to the pump shutting off (mpg calculation).

Measuring with a 100 ft tape that is actually 99.01 ft.

Press the wrong key(s) on a calculator during a long calculation.

Definitions There are three ways to numerically

describe error:

1) True error = Reported value - True value

2) Fractional error = True error / True value

3) Percent error = Fractional error * 100%

An example:Calculate the true error, fractional error and

percent error for:Reported Value

Distance

feet

Correct Value

Distance

feet

109 100

1050 1000

45 50

17 20