Authors
Kevin Potcner
JMP
Objective
Evaluate the durability of mobile phone screens in a drop test at various drop heights. Determine if a desired level of durability is achieved for each of three types of screens and compare performance.
Background
The durability of a product is clearly an important quality characteristic for both the end user and the manufacturer. For end users, durability is especially important for mobile phones. Dropping a phone on a hard surface, for example, can result in the screen cracking or even breaking, rendering the phone unusable. To evaluate the durability of these screens, manufacturers subject a sample of screens to a variety of tests to simulate typical wear and tear by a user, such as dropping the phone onto a concrete surface.
In JMP032 Durability of Mobile Phone Screen - Part 1, material scientists for a screen manufacturer experimented with two new formulations of an aluminosilicate glass (A and B). These two formulations were produced by making a change to a final processing step that uses a specific level of potassium nitrate to strengthen the glass.
A sample of 10 screens of each type was developed for testing. Each screen was installed into the same style of phone. The phones were then dropped in a controlled identical manner from a height of 1 meter onto a concrete surface. A binary variable “Success” (no damage) and “Fail” (screen damage) was recorded.
One of the company’s goals is that 97% of the screens manufactured would be able to experience a drop of 1 meter without becoming damaged (i.e., the Population Success Rate).
The analyses illustrated in JMP032 Durability of Mobile Phone Screen - Part 1, which were based on only 10 phones tested of each Screen Type, failed to generate the statistical evidence needed to demonstrate the desired Success Rate. The analyses also failed to find any statistically significant difference between the two Screen Types.
The engineers decided to test an additional 40 phones for each of the two Screen Types. Analyzing the results from this expanded test was the objective of the exercises for that case study.
To continue the work in creating a highly durable material to use for mobile phone screens, the engineers developed a third formulation (Type C), which requires a more expensive process. In the test, 50 specimens of Type C were subjected to a 1.0m drop. In an effort to gain a more comprehensive understanding of the durability of these screens, the engineers also conducted the drop test at two additional heights (0.5 and 1.5 meters). At 1.5m, 50 phones of each Screen Type were tested; 40 were used for the 0.5m test. These new tests, along with the previous tests, provide us with the following data for analysis:
| 0.5m | 1.0m | 1.5 | |
| Type A | n=40 S=36 F=4 | n=50 S=41 F=9 | n=50 S=28 F=22 |
| Type B | n=40 S=40 F=0 | n=50 S=48 F=2 | n=50 S=40 F=10 |
| Type C | n=40 S=39 F=1 | n=50 S=47 F=3 | n=50 S=39 F=11 |
The Task
The primary objectives of this analysis are:
- 1. Describe the durability of each of the three Screen Types at the three different Drop Heights.
- 2. Describe any statistical differences across the Screen Types and Drop Heights.
- 3. Determine if the durability of Screen Type C warrants the more expensive process (i.e., is the durability of Screen Type C better than that of A and B?).