7QC Tools: Scatter PlotAmrendra Roy
In general after preparing flow chart there is a brainstorming session, and the outcome of this brainstorming session is the fish-bone diagram which in-turn list down the probable parameters or causes that can affect your quality parameter. At this step all parameters thus collected are only suspects (unless proven guilty!) and their role in affecting the quality parameters needs to be proved in order to held them guilty. In this regard historical data of all suspects (probable parameters or causes) are collected and their effect on the quality parameter is evaluated using scatter plot.
This evaluation can be dome by expert using ANOVA, Regression etc. but a visual tool was required which can tell a shop-floor person that a given parameter is affecting the quality attributes or not? In this regard scatter plot comes handy where you plot a given parameter against the quality attributes.
In real case scenario, we end-up with huge data base as shown below, where X1 to X8 represents the process parameters (suspects) and y represents the quality attribute of interest.
By visual inspection of the above data, it becomes difficult to analyze the effect of X on Y, situation becomes worse if data is larger. Hence, scatter plot is a visual tool that gives qualitative correlation between X and Y.
Let’s look at the scatter plot of X1 and X2 Vs. Y
X-Axis represents X and Y-axis represents Y. we can see that in scatter plot of X1 Vs. Y, as the value of X1 increases, Y also increases. Whereas in X2 Vs. Y, there is no apparent correlation. Hence, we can conclude that X1 is affecting the Y and there is no effect of X2 on Y. We can also quantify the effect by calculating R2 values. R2 values can vary from -1 to +1. The values close to +1 indicates strong positive correlation whereas values close to -1 indicates strong negative correlation.
Scatter plot of all X Vs. Y are given below along with R2 values.
Looking at the scatter plot given above, any shop-floor person can tell that X1, X3 and X5 are affecting the Y in positive way whereas, X4 and X8 are affecting the Y in negative way. Also X2 and X6 doesn’t have any effect on Y.