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Design and Analysis of Multi-Factored Experiments. Fractional Factorial Designs. Design of Engineering Experiments – The 2 k-p Fractional Factorial Design.

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design and analysis of multi factored experiments

Design and Analysis ofMulti-Factored Experiments

Fractional Factorial Designs

DOE Course

design of engineering experiments the 2 k p fractional factorial design
Design of Engineering Experiments – The 2k-p Fractional Factorial Design
  • Motivation for fractional factorials is obvious; as the number of factors becomes large enough to be “interesting”, the size of the designs grows very quickly
  • Emphasis is on factorscreening; efficiently identify the factors with large effects
  • There may be many variables (often because we don’t know much about the system)
  • Almost always run as unreplicated factorials, but often with center points

DOE Course

why do fractional factorial designs work
Why do Fractional Factorial Designs Work?
  • The sparsity of effects principle
    • There may be lots of factors, but few are important
    • System is dominated by main effects, low-order interactions
  • The projection property
    • Every fractional factorial contains full factorials in fewer factors
  • Sequential experimentation
    • Can add runs to a fractional factorial to resolve difficulties (or ambiguities) in interpretation

DOE Course

the one half fraction of the 2 k
The One-Half Fraction of the 2k
  • Notation: because the design has 2k/2 runs, it’s referred to as a 2k-1
  • Consider a really simple case, the 23-1
  • Note that I =ABC

DOE Course

the one half fraction of the 2 3
The One-Half Fraction of the 23

For the principal fraction, notice that the contrast for estimating the main effect A is exactly the same as the contrast used for estimating the BC interaction.

This phenomena is called aliasing and it occurs in all fractional designs

Aliases can be found directly from the columns in the table of + and - signs

DOE Course

the alternate fraction of the 2 3 1
The Alternate Fraction of the 23-1
  • I = -ABC is the defining relation
  • Implies slightly different aliases: A = -BC, B= -AC, and C = -AB
  • Both designs belong to the same family, defined by
  • Suppose that after running the principal fraction, the alternate fraction was also run
  • The two groups of runs can be combined to form a full factorial – an example of sequential experimentation

DOE Course

slide7
Example: Run 4 of the 8 t.c.’s in 23: a, b, c, abc

It is clear that from the(se) 4 t.c.’s, we cannot estimate the 7 effects (A, B, AB, C, AC, BC, ABC) present in any 23 design, since each estimate uses (all) 8 t.c’s.

What can be estimated from these 4 t.c.’s?

DOE Course

slide8
4A = -1 + a - b + ab - c + ac - bc + abc

4BC = 1 + a - b - ab -c - ac + bc + abc

Consider

(4A + 4BC)= 2(a - b - c + abc)

or

2(A + BC)= a - b - c + abc

Overall:

2(A + BC)= a - b - c + abc

2(B + AC)= -a + b - c + abc

2(C + AB)= -a - b + c + abc

In each case, the 4 t.c.’s NOT run cancel out.

DOE Course

slide9
Had we run the other 4 t.c.’s:

1, ab, ac, bc,

We would be able to estimate

A - BC

B - AC

C - AB

(generally no better or worse than with + signs)

NOTE: If you “know” (i.e., are willing to assume) that all interactions = 0, then you can say either (1) you get 3 factors for “the price” of 2.

(2) you get 3 factors at “1/2 price.”

DOE Course

slide10
Suppose we run those 4:

1, ab, c, abc;

We would then estimate

A + B

C + ABC

AC + BC

In each case, we “Lose” 1 effect completely, and get the other 6 in 3 pairs of two effects.

Members of the pair are CONFOUNDED

Members of the pair are ALIASED

two main effects

together usually

less desirable

DOE Course

slide11
With 4 t.c.’s, one should expect to get only 3 “estimates” (or “alias pairs”) - NOT unrelated to “degrees of freedom being one fewer than # of data points” or “with c columns, we get (c - 1) df.”

In any event, clearly, there are BETTER and WORSE sets of 4 t.c.’s out of a 23.

(Better & worse 23-1 designs)

DOE Course

slide12
Prospect in fractional factorial designs is attractive if in some or all alias pairs one of the effects is KNOWN. This usually means “thought to be zero”

DOE Course

slide13
Consider a 24-1 with t.c.’s

1, ab, ac, bc, ad, bd, cd, abcd

Can estimate: A+BCD

B+ACD

C+ABD

AB+CD

AC+BD

BC+AD

D+ABC

- 8 t.c.’s

-Lose 1 effect

-Estimate other 14 in 7 alias pairs of 2

Note:

DOE Course

slide14
“Clean” estimates of the remaining member of the pair can then be made.

For those who believe, by conviction or via selected empirical evidence, that the world is relatively simple, 3 and higher order interactions (such as ABC, ABCD, etc.) may be announced as zero in advance of the inquiry. In this case, in the 24-1 above, all main effects are CLEAN. Without any such belief, fractional factorials are of uncertain value. After all, you could get A + BCD = 0, yet A could be large +, BCD large -; or the reverse; or both zero.

DOE Course

slide15
Despite these reservations fractional factorials are almost inevitable in a many factor situation. It is generally better to study 5 factors with a quarter replicate (25-2 = 8) than 3 factors completely (23 = 8). Whatever else the real world is, it’s Multi-factored.

The best way to learn “how” is to work (and discuss) some examples:

DOE Course

construction of a one half fraction
Construction of a One-half Fraction

The basic design; the design generator

DOE Course

slide18
Example: 25-1 : A, B, C, D, E

Step 1: In a 2k-p, we “lose” 2p-1.

Here we lose 1. Choose the effect to lose. Write it as a “Defining relation” or “Defining contrast.”

I = ABDE

Step 2: Find the resulting alias pairs:

*A=BDE AB=DE ABC=CDE

B=ADE AC=4 BCD=ACE

C=ABCDE AD=BE BCE=ACD

D=ABE AE=BD

E=ABD BC=4

CD=4

CE=4

- lose 1

- other 30 in 15 alias pairs of 2

- run 16 t.c.’s

15 estimates

*AxABDE=BDE

DOE Course

slide19
See if they are (collectively) acceptable.

Another option (among many others):

I = ABCDE

A=4 AB=3

B=4 AC=3

C=4 AD=3

D=4 AE=3

E=4 BC=3

BD=3

BE=3

CD=3

CE=3

DE=3

DOE Course

slide20
Next step: Find the 2 blocks (only one of which will be run)
  • Assume we choose I=ABDE

I II

1 c a ac

ab abc b bc

de cde ade acde

abde abcde bde bcde

ad acd d cd

bd bcd abd abcd

ae ace e ce

be bce abe abce

Same process

as a

Confounding

Scheme

DOE Course

slide21

1

a

b

ab

c

ac

bc

abc

e

ae

be

abe

ce

ace

bce

abce

Next: Pick which block to run.

(say, block II)

Next: Go out and collect the data.

Next: Analyze it.

a.) find a proper Yates’ order.

i.) pick a letter and for a moment

call it “DEAD.” (assume we pick “d”)

ii.) use the remaining (“live”) letters

to form the STANDARD Yates’ order:

(see right column)

Now append the dead letter as needed to form

the block chosen to be run:

DOE Course

slide22

There’s only one way to do it, i.e. either adding “d” or not adding “d”; one way will work, one won’t.

DOE Course

slide23
Example 2:

25-2 A, B, C, D, E

Must “lose” 3; other 28

in 7 alias groups of 4

In a 25 , there are 31 effects; with 8 t.c., there are 7 df & 7 estimates available

DOE Course

slide24
Choose the 3: Like in confounding schemes, 3rd

must be product of first 2:

I = ABC = BCDE = ADE

A = BC = 5 = DE

B = AC = 3 = 4

C = AB = 3 = 4

D = 4 = 3 = AE

E = 4 = 3 = AD

BD = 3 = CE = 3

BE = 3 = CD = 3

Assume we use this design.

Find alias groups:

DOE Course

slide25

Let’s find the 4 blocks: I =ABC = BCDE = ADE

Assume we run the Principal block (block 1)

DOE Course

slide26
Run the 8 t.c.’s and analyze: Since it’s a 25-2, we designate 2 letters as “DEAD” (say b, d), write a standard Yates’ order in the other (3) (live) letters, and append the dead letters to form the t.c.’s being run:

I = ABC = BCDE = ADE

t.c.

yield

(1)

(2)

(3)

Estimate

1

.

-

.

a (

bd)

-2+5-2

A

.

c (b)

-2+3-4

C

.

ac (d)

AC - +4-3

B

e (d)

.

-4+3-2

E

D

.

ae (b)

AE -3+4-

.

ce (

bd)

CE -3+2-3

.

ace

ACE -2+3-2

DOE Course

slide27
Good to know rule:

t.c. with even # letters in common with even-lettered effect is a + for that effect; t. c. with odd # letters in common with odd-lettered effect is a + for that effect; otherwise a - (minus)

abd in ABCDE: 3 in common with 5

ODD with ODD +

abce in ABCDFG: 3 in common with 6

ODD with EVEN -

# letters in effect

Even

Odd

# letters t.c. has in

Common with Effect

Even

Odd

DOE Course

examples
Examples

DOE Course

example
Example

Interpretation of results often relies on making some assumptions

Ockham’srazor

Confirmation experiments can be important

See the projection of this design into 3 factors

DOE Course

projection of fractional factorials
Projection of Fractional Factorials

Every fractional factorial contains full factorials in fewer factors

The “flashlight” analogy

A one-half fraction will project into a full factorial in any k – 1 of the original factors

DOE Course

possible strategies for follow up experimentation following a fractional factorial design
Possible Strategies for Follow-Up Experimentation Following a Fractional Factorial Design

DOE Course

the one quarter fraction of the 2 6 2
The One-Quarter Fraction of the 26-2

Complete defining relation: I = ABCE = BCDF = ADEF

DOE Course

analysis of fractional factorials
Analysis of Fractional Factorials
  • Easily done by computer
  • Same method as full factorial except that effects are aliased
  • All other steps same as full factorial e.g. ANOVA, normal plots, etc.
  • Important not to use highly fractionated designs - waste of resources because “clean” estimates cannot be made.

DOE Course

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