Overcoming Difficulties with Applying
Traditional DOE to Reliability Data
Traditionally, there have
been some difficulties with employing DOE analysis methods to
properly analyze the response information that is often of
interest to reliability engineers — product lifetime data.
Specifically, traditional
DOE analysis techniques such as ANOVA and standard linear
regression are based on the assumptions that the data set will
be complete and normally distributed. However, the life data
sets commonly used for reliability-related analyses may include
information about items that did not fail (right censored data)
and/or uncertainty about the time of the failure (interval
data). In addition, life data sets often follow a skewed
distribution such as the Weibull, lognormal or exponential.
Therefore, in order to apply traditional analysis techniques to
life data that do not fit the assumptions, analysts have had to
adjust the interval and right censored points in their data sets
(usually by treating the suspension times as failures and using
the mid-points of the intervals as the failure times). Then if
the adjusted data set was not normally distributed, they also
had to apply a logarithmic transformation before analyzing the
data. Unfortunately, the adjustments and transformations
required to force the life data to fit the assumptions could
result in erroneous conclusions from the analysis.
Now there is an
alternative approach available only in
DOE++
— reliability DOE!
Reliability DOE Provides Proper Treatment for
Life Data
Reliability DOE is a
special category of DOE where traditional experiment design
types are combined with reliability analysis methods to
investigate the effects of different factors on the life of a
unit. Instead of using the F ratio (as in traditional DOE techniques), reliability
DOE uses the likelihood ratio, which provides the appropriate
treatment for interval and right censored data and supports
analysis with skewed distributions, including the Weibull,
lognormal and exponential — no adjustments or transformations
required!
The reliability DOE designs
available in
DOE++
include:
One Factor
Reliability Designs
Two Level Full
Factorial Reliability Designs
Two Level
Fractional Factorial Reliability Designs
Plackett-Burman
Reliability Designs
Modify Design Window
The Modify Design window allows you to change an
existing design in a variety of ways. Depending on the type of
design you are working with, this may include the ability to add
replicates, fold the design, add center points and/or add an
axial block. This feature makes it easy to move from a basic
analysis to a more in-depth look at your data without having to
initiate a new analysis using the Design Wizard.
File Import Wizard
The File Import Wizard
makes it easy for you to create new designs in
DOE++
using experimental data from files saved by other applications.
You can import data from:
Excel files (*.xls)
Tab, comma, space
and semicolon delimited files (*.txt, *.csv, *.prn, *.smc)
Simply use the Wizard to
open your existing file, select the type of design you want to
create and specify the type of data contained in each column
within the file. Your data will be transferred instantly to a
new folio in the format required for analysis in
DOE++.
