Samples or groups for comparison Controls Samples or groups for comparison

Positive Control Sample that represents the typical or unaltered procedure “Standard” Predictable, reliable outcome Verifies that the procedure, materials and equipment are all working correctly

Negative Control Sample that leaves out the independent variable Outcome should be predictable Verifies that the independent variable is actually responsible for differences seen in the experimental samples

Validity Based on the outcomes of the positive and negative controls If the controls give the expected results, the experiment is valid

Reliability A procedure or protocol gives consistent, similar results Determined by repeated trials yielding the same results each time Not necessarily dependent on controls or validity

Experimental Error The difference between the “true” value and the measured or estimated value Due to limitations of accuracy and precision Does NOT include mistakes (if you make a mistake, redo it!)

Systematic Error Error is consistent and repeatable Often due to equipment being miscalibrated or a mistyped or misinterpreted procedure

Random Error Under the same conditions, a different value is measured every time Unpredictable and difficult to explain