All statistics Create Definitive Screening Design

Find definitions and interpretation guidance for every statistic that is provided with the creation of a definitive screening design.

Factors

The number shows how many factors are in the design.

Interpretation

The factors are the variables that you control in the experiment. Factors are also known as independent variables, explanatory variables, and predictor variables. Factors assume only a limited number of possible values, known as factor levels. Factors can have text or numeric levels. For numeric factors, you select specific levels for the experiment, even though many values for the factor are possible. For categorical factors, you can have only two levels.

For example, you are studying factors that could affect plastic strength during the manufacturing process. You include factors for additive and temperature in the experiment. The additive is a categorical variable. Additive can be type A or type B.

Temperature is a continuous variable. Because temperature is a numeric factor, 3 temperature settings are in the experiment. The low level is 100°C. The high level is 200°C. The midpoint of these two levels is also in the design, 150°C.

Replicates

The number shows how many replicates are in the design.

Interpretation

Replicates are multiple experimental runs with the same factor settings (levels). One replicate is equivalent to the base design, where you conduct each factor level combination once. With two replicates, you perform each factor level combination in the base design twice (in random order), and so on.

For example, if you have 4 factors, the base design represents 1 replicate and has 13 runs. If you add 2 replicates, the design includes 3 replicates and has 39 runs.

When planning your experiment, consider the following when you decide whether to add replicates:
  • If you are trying to create a prediction model, multiple replicates can increase the precision of your model.
  • If you include replicates, you might be able to detect smaller effects because you will have greater power to detect an effect of fixed size.
  • Screening designs, which are used to reduce a large set of factors, usually don't include replicates initially. Screening designs include Plackett-Burman designs, definitive screening designs, and some 2-level factorial designs.
  • Your resources can dictate the number of replicates you can run. For example, if your experiment is extremely costly, you might be able to run the base design only one time.

For information on the difference between replicates and repeats, go to Replicates and repeats in designed experiments.

Base runs

The number of base runs is the number of unique factor level combinations in the base design. The total number of runs is the number of base runs times the number of replicates.

Interpretation

Use the number of base runs to identify the design. A run is an experimental condition or factor level combination at which responses are measured. Each run corresponds to a row in the worksheet and results in one or more response measurements, or observations. For example, you do a full factorial design with two factors, each with two levels. Your experiment has four runs:

The base runs are the initial design, or starting point, from which Minitab can build the final design. You can add replicates, which then add runs to the base number of runs. For example, you create an 8-factor definitive screening design. The base number of runs is 17. With 2 replicates, the total number of runs is 34.

Run Factor 1 Factor 2 Response
1 -1 -1 11
2 1 -1 12
3 -1 1 10
4 1 1 9
5 1 -1 8
6 1 1 12
7 -1 1 10
8 -1 -1 11
Note

When doing an experiment, the run order should be randomized.

Each run corresponds to a design point, and the entire set of runs is the design. Multiple executions of the same experimental conditions are considered separate runs and are called replicates.

Total runs

The total number of runs is the number of base runs times the number of replicates. The total number of runs equals the number of rows in the worksheet.

Interpretation

Use the number of total runs to verify that the experiment is the right size for your resources. A run is an experimental condition or factor level combination at which responses are measured. Usually, each run corresponds to a row in the worksheet and results in one or more response measurements, or observations. For example, you do a full factorial design with two factors, each with two levels and have two replicates. Your experiment has eight runs:
Run Factor 1 Factor 2 Response
1 -1 -1 11
2 1 -1 12
3 -1 1 10
4 1 1 9
5 1 -1 8
6 1 1 12
7 -1 1 10
8 -1 -1 11
Note

When doing an experiment, the run order should be randomized.

Each run corresponds to a design point, and the entire set of runs is the design. Multiple executions of the same experimental conditions are considered separate runs and are called replicates.

Base blocks

The number shows that the base design always has 1 block. If you have multiple replicates, the total number of blocks can be greater than the base number of blocks.

Interpretation

Blocks account for the differences that might occur between runs that are performed under different conditions. For example, an engineer designs an experiment to study welding and cannot collect all of the data on the same day. Weld quality is affected by several variables that change from day-to-day that the engineer cannot control, such as relative humidity. To account for these uncontrollable variables, the engineer groups the runs performed each day into separate blocks. The blocks account for the variation from the uncontrollable variables so that these effects are not confused with the effects of the factors the engineer wants to study.

Total blocks

The number shows how many blocks are in the design.

Interpretation

Blocks account for the differences that might occur between runs that are performed under different conditions. For example, an engineer designs an experiment to study welding and cannot collect all of the data on the same day. Weld quality is affected by several variables that change from day-to-day that the engineer cannot control, such as relative humidity. To account for these uncontrollable variables, the engineer groups the runs performed each day into separate blocks. The blocks account for the variation from the uncontrollable variables so that these effects are not confused with the effects of the factors the engineer wants to study.

Center Points

The number shows how many center points are in the design.

Interpretation

Center points are runs where all numeric factors are set at the midpoint between their low and high levels. For example, if a numeric factor has levels 100 and 200, the midpoint is 150. Definitive screening designs with all continuous factors have 1 center point per replicate. If you have text factors, Minitab adds 2 center points per replicate.

Design table

The design table shows the factor settings for each experimental run. Because the design table takes up less space than the worksheet, it can be useful for reports with limited space.

Letters represent the factors and follow the order that you used when you created the design. In each row, − indicates that the factor is at the low setting and + indicates that the factor is at the high setting. A 0 indicates that a numeric factor is midway between the low and high settings.

Interpretation

Use the design table to see the factor settings for each run and the order of the runs in the design. In these results, the design table shows the experimental conditions or settings for each of the factors for the design points. The run order is random. For example, in the first run of the experiment, Factors B, E, and F are at the high setting. Factors A, C, D, and G are at the low setting. Factor H is at the middle setting. The design includes 2 center points, which are runs 12 and 34.

Definitive Screening Design

Design Summary Factors: 8 Replicates: 2 Base runs: 17 Total runs: 34 Base blocks: 1 Total blocks: 1 Center points: 2
Design Table (randomized) Run Blk A B C D E F G H 1 1 0 + + + + + + + 2 1 0 0 0 0 0 0 0 0 3 1 + - - + 0 + - + 4 1 - - + - - + 0 + 5 1 + + - - - 0 + + 6 1 - + 0 + - - - + 7 1 + - + + - - + 0 8 1 + - 0 - + + + - 9 1 - - + - - + 0 + 10 1 + 0 + - + - - + 11 1 0 0 0 0 0 0 0 0 12 1 - + + - 0 - + - 13 1 0 - - - - - - - 14 1 - + + - 0 - + - 15 1 + - + + - - + 0 16 1 - - - 0 + - + + 17 1 0 - - - - - - - 18 1 - + 0 + - - - + 19 1 + + - + + - 0 - 20 1 + + + 0 - + - - 21 1 + - 0 - + + + - 22 1 - 0 - + - + + - 23 1 - - + + + 0 - - 24 1 + + - - - 0 + + 25 1 + - - + 0 + - + 26 1 + 0 + - + - - + 27 1 - + - - + + - 0 28 1 + + - + + - 0 - 29 1 - - + + + 0 - - 30 1 - 0 - + - + + - 31 1 0 + + + + + + + 32 1 - + - - + + - 0 33 1 - - - 0 + - + + 34 1 + + + 0 - + - -
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