CHAPTER 1 : ACCURACY & CONSISTENCY

Accuracy and Consistency in Measurements 1.4.1  Accuracy 1.     ___________  of a measurement is how close the value of a measurement         to the ____________ value. 2.     The level of accuracy is related to the relative error.                                                 3.     An error is a                      between the measured value and the actual         value  . 4.     Accuracy can be improved by  : -             (a)   repeated readings are taken and the average value is calculated              (b)   avoid parallax errors             (c)   avoid zero errors             (d)   use measuring instruments with a higher accuracy.             For example, a vernier  callipers is more accurate than a ruler

1.4.2  Consistency 1.                           / Precision of an instrument is the ability of the instrument         to measure a quantity with little or no deviation among measurements.                                 2..    A measurement is considered consistent  will have a              relative         deviation or no deviation from the  average value . 3.    A deviation is a difference between a  _______________value and its        average value .                                                       4.     average deviation   =          Total deviation                                              __________________                                             number of value taken                                                 5.     relative deviation   =   average deviation    x  100  %                                            ______________                                               average value 6.     Consistency can be improved by             (a)  eliminating parallax errors            (b)  exercising greater care and effort when taking readings.             (c)  using an instrument which is not defective. 7.          Comparisons between  consistency and  accuracy by considering a  target.

Example: A student used vernier callipers to measure diameter of a glass rod. The table below shows the readings. Measurement Diameter rod (cm) Deviation 1 2.23 0.012 2 2.26 0.018 3 2.24 0.002 4 2.23 0.012 5 2.25 0.008 Average 2.242 0.01 Relative deviation        = ( 0.01 / 2.242) x 100%                                     = 0.45 %

1.4.3  Sensitivity in Measurements 1. Sensitivity of an instrument is its ability to detect a small change in the     quantity to be measured. 2. A measuring instrument that has a scale with a smaller  divisions is more    sensitive . 3. Measuring  instruments.             Measuring instruments Smallest magnitude of quantity (cm) Sensitivity / Accuracy Metre rule 0.1 0.1 cm (low) Vernier callipers 0.01 0.01 cm (moderate) Micrometre screw gauge 0.001 0.001 cm (high)

1.4.4	Errors in Measurements
	1. An error is the difference between the measured value and the actual     value. 2. There are two main types of errors in measurements,     A.    systematic errors     B.    random errors. A. Systematic  errors     i.      The error in calibration of instrument which makes the instrument             defective. (We must examine the instrument carefully before using             them)                   ii.                   ________ error which means the pointer of the instrument does not         return to zero when not in use. ( Zero error can be corrected by            compensating the readings)                                         iii.    A problem which persists throughout  the experiment such as             repeated error in reaction time and wrong assumption.          Systematic errors will lead to decrease in
	B. Random errors     i.     Arise from unknown and unpredictable variations in condition, and            will produce a different error every time you repeat the experiment.     ii.    May be due  to:            (a)  personal  error  (human limitations of  sight and touch)            (b)  lack of sensitivity  ( instrument does not respond  / indicate                  insignificant or small change )            (c)  natural errors  ( wind , temperature, humidity, refraction, magnetic                  Field or gravity )            (d)  wrong  technique  ( applying excessive pressure when                  turning a micrometer screw gauge )      iii.   Can be minimized by repeating the measurement several times              and taking the average (mean) of the reading .     iv.   Parallax error           An error in reading a measurement because an observer’s eye and           the pointer are not in a line perpendicular to the plane of the scale .           (the eyes of the observer must be align and perpendicular to the scale of            measuring instrument)