Question

1. A local food delivery company “quiksnak” wants to improve its process to provide better customer service. This is particularly important for the company because competitors like Doordash and Postmates are providing consistency in delivery times. The operations manager collected the most recent data on delivery times. For each of their 4 delivery hubs, they collected 8 delivery times (8 samples) to check process variability. All the data shown is measured in minutes.

Sample #	1	2	3	4	5	6	7	8
Hub #1	21.1	20.2	24.1	22.6	21.2	20.9	22.4	20.9
Hub #2	21.7	20.6	21.7	21.7	24.1	23.5	24.6	24.7
Hub #3	23.0	24.5	23.2	23.0	20.6	22.8	23.4	22.8
Hub #4	20.3	23.5	24.4	25.0	20.7	23.2	23.3	24.5

Use the data to check if the process they have currently is “in control”. You will use and draw control charts to answer this question.              (12 pts.)

2. For question 2, what type of control chart did you choose and why? Please refer to Xbar-Chart and p-chart differences when answering this question. (5 pts.)        

Answer 1

Answer 1

Using the given data two control charts are drawn. Xbar-chart and R-chart.

From the two charts it is concluded that the process they have currently is in control. Because in both the charts, data points are within the upper control limit and lower control limit in a satisfactory level.

All the calculations and two charts are given in the images below:

Answer 2

The Control charts used are Xbar-chart and R-chart. These are the most commonly used variable control charts. Variable control charts are the one which is used for measurable quality characteristics. Xbar chart shows the centring of process,i.e. it shows the variation in the averages of samples. R-chart shows the uniformity or consistency of the process,i.e. it shows variation in the ranges of samples.

• Xbar and R chart is used for quality characteristic that can be measured and expressed in numbers but P chart is an attribute control chart,i.e. for quality characteristic that can be classified as either conforming to the specifications. So here using Xbar and R is more appropriate.
• Xbar and R chart is best suited for critical dimendions. P chart is best suited in cases where inspection is carried out with a view to classifying an article as accepted or rejected.
• The sample size is generally larger for P chart than for Xbar and R chart. The variations in the sample sizes influences the control limits much more in Xbar and R charts than in P chart.

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