Now coming to the sixth tool which is Control Charts, control charts are basically line charts 2
depending on time. 3
So I take measurement and I plot those measurement over a period of time. In addition to the normal 4
line curve 5
these have upper and lower control limits also. These upper and lower control limits are calculated based 6
on the data which I collect. 7
So if I just take an example of my reaching office at 6:30 if I have taken the reading of every day 8
like day one I reached at 6:10, day two I reached at 6:15, day three I reach at 6:40. 9
All these things if I plot along a line, along a time series and I calculate the upper and lower control 10
limit based on the formulas. 11
I'm not going into the details of those formulas here. 12
There are a number of types of control charts. 13
I'm not going into the details of that as well. 14
I'm just covering control chart at a high level that in-control chart you have mean, you have upper control 15
limit, you have lower control limit. 16
You have number of readings ploted along with the time that's how a control 17
chart will look like. If during this process if any point which falls above the upper control limit or below 18
the lower control limit then you can predict that something wrong has happened, something assignable has 19
happened. Earlier when we talked about quality gurus 20
there we talked about Demming as well. 21
Demming has emphasized a lot on special causes and common causes. 22
People need to know about special cause and common cause. 23
Common Cause is a normal fluctuation. 24
If I go to office planned for 6:30 I might go two minutes early two minutes late depending on traffic, 25
depending on signals. 26
All those factors might lead to some fluctuation. 27
That's a common cause that's built in to the system. 28
You really cannot do anything unless you make a drastic change to the process. 29
But then in addition to common causes there are special causes also. 30
Let's say one day there was an accident on the road, the road was blocked. 31
And that took me another 20-30 minutes to wait. 32
And that day I reached at seven o'clock. In my control chart that point will be shown much above the upper 33
control limit. 34
So if I see a point above the upper control limit I can judge that something wrong has happened. 35
There is some assignable cause and in that case I need to investigate, look into that assignable cause 36
and eliminate that from that system. 37
That's how you do 38
for any process. Be it reaching at any time in the office or be the volume control in case of that water 39
bottle or any other process whatever you are doing. 40
Once you see that you'll have some reading which is abnormal, which is above the upper control limit, 41
below the lower limit. 42
You need to do some investigation and how do you do investigation. 43
You go from cause and effect diagram. 44
You plot a diagram. 45
See that what all could have caused that. 46
So here you would see that all these tools are interrelated. 47
So you find a problem with the control chart. 48
You go to the cause and effect diagram and look at all the causes. 49
So these tools work in combination with each other. 50
Now coming back to control chart. 51
That's what we were discussing here. 52
Why do we need control chart, we need control chart to identify when things go wrong in the process. 53
Identify when there is a special cause. 54
And this also helps in telling operator when to take action and when not to take action because if things 55
are running within control limit, if things go up and down and there is no need to take any action because 56
if you take too many action, if you fiddle with the system too much then it's basically going to spoil, 57
basically going to make things worse. 58
So if the changes in the process are just because of Common Cause you really don't need to adjust 59
thing. 60
For example let's go back to the same example of going to office like if I'm late for two minutes 61
one day. 62
That doesn't mean that next day I should start two minutes early because this is a natural fluctuation. 63
There is a natural fluctuation in any process. 64
That's something which we need to identify and that's where control chart 65
help us. 66
Now let's look at the example on the next slide.