Basic tools of quality

Basic tools of quality

1. Control charts

   • It basically consists of the data of a process captured chronologically in a graph with established limits.
   • Types
     • Control chart for variables
       • Graph X
         • Plot the process mean over time. Used to track the level of the process and detect the presence of special causes
       • Graph R
         • Plot the rate of the process over time. Used to track process variation and detect unexpert variation.
       • Graph X-R
         • We use both types of plots when measuring the relationship of quality specifications with central tendency and dispersion.
       • Zone graph
         • Plots cumulative scores based on "zones" at 1, 2, and 3 standard deviations from the center line. Use it to detect an unexpected variation.
       • Graph S
         • It is used to monitor the variation (standard deviation) of the process when you have continuous data and subgroup sizes of 9 more. This chart is used to monitor process stability over time so that you can identify and correct instablities in a process.
     • Control charts by attributes
       • Graph P
         • In it we measure the percentage of defects per sample.
       • Graph NP
         • Unlike p, this value is not a fraction. It is the number of defective units in a sample.
       • Graph C
         • It is the number of defects per unit of production during a sampling period.
       • Graph U
         • In it we measure the percentage of defects in a unit during a sampling period.

2. Frequency distribution

   • It is the way in which a series of observations is ordered in differents groups, and normally in ascending or descinding mode.
   • Types
     • Absolute frequency
       • Itis the number of observations that belong to each group.
     • Relative frequency
       • It is calculated by diving the absolute frequency by the number of data.
     • Accumulatived absolute frequency
       • It results from adding the absolute frequencies of a class or group of the sample with the previous one or previous ones.
     • Cumulative relative frequency
       • It is result of adding the relative frequencies

3. Stastics and parameters

   • Statistical
     • It is a numerical quality calculated on the sample that summarizes its information on some aspect. It is used to approximate a parameter.
   • Parameter
     • It is a numerical quantity calculated on a population and summarizes the values that this takes on some attribute.

4. Parameters estimation

   • Parameter estimation is the procedure used to determine the characteristics of population parameter, based on knowledge of the same.

5. Measures of central tendency and dispersion

   • Measures of central tendency
     • Measures of central tendency are statical measures that aim to summarize a set of values in a single value.
   • Measures of dispersion
     • The measures of dispersion measure the degree of dispertion of the values of the variation.