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RD-Ri47 9i5 CONJUNCTIVE AND DISJUNCTIVE ITEMI RESPONSE FUNCTIONS(U) i/i EDUCATIONAL TESTING SERVICE PRINCETON NJ F MI LORD OCT 84 N@@@i4-83-C-8457 UNCLSSIFIED F/0 12/ NL mhhhhhhmhmml

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RD-Ri47 9i5 CONJUNCTIVE AND DISJUNCTIVE ITEMI RESPONSE FUNCTIONS(U) i/iEDUCATIONAL TESTING SERVICE PRINCETON NJ F MI LORDOCT 84 N@@@i4-83-C-8457

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Cojncieand Disjunctive Item Response Technical ReportFunctions 5 sraaa u.uormml

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Fredeic N.Lord 00014-83-C-0457

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Educational Testing ServicePrinceton, NJ 08541 NR 150-520

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Item Response TheoryFunctional Equations

MO LOSYRAC? CMUSUOt sus "WOMad MI UfeW NE OW Moe or OlAMMGiven that the examinee knows the answer to item i if and only if he

knows the answer to both item g and item h , a 'conjunctive' item responsemodel is found such that items g , h , and i all have the same mathemati-cal form of response function. Since such items may occur in practice, it isdesirable that item response models satisfy this condition. For models withtwo parameters per item, the most general functional form satisfying thiscondition is found. A third, 'guessing' parameter may be added. The cor-

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SN ofa103. 6% *1*. &do I 561,MIVpAIIICAVION OF 7365 -paS (l e.-

Ites Response Functions "

I

Conjunctive and Disjunctive Item Response Functions

Frederic M. Lord

Educational Testing Service

October 1984

Accession For

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Availability CodesAvail and/or

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* Item Response nctions

2

Conjunctive and Disjunctive Item Response Functions

Abstract

Given that the examinee knows the answer to Item i if and

only if he knows the answer to both Item g and item h , a

'conjunctive' Item response model Is found such that items g , h

and i all have the smine mathematical form of response function.

Since such items may occur in practice, it is desirable that item

response models satisfy this condition. For models with two

parameters per item, the most general functional form satisfying

this condition is found. A third, 'guessing' parameter may be

added. The corresponding disjunctive model is also derived.

' .''.

• .1

5-.

."' ft.;z),'o

item Response Functions

3

Conjunctive and Disjunctive Item Response Functions*

Consider two free-response spelling items that ask the

examinee to spell, respectively, the word wife and the word house.

Now consider a free-response spelling item that asks the examinee

to spell the word housewife. Assume that an examinee will answer

the third item correctly if and only If the examinee would answer

both of the first two items correctly. If so, then

P3 (6) = PO)P2() (1)

where 8 is the ability of the examinee and P1(0) is the item

response function for (probability of giving a correct answer to)

item I •

Kristof (1968) pointed out that it is highly desirable that

P , and P3 should all have the same mathematical form,

but that this condition is not satisfied by the usual logistic or

normal ogive models In Item response theory (IRT). He derived one-

parameter families of Item response function, P(6) , satisfying

(1), stating that this result "is attainable If and only If all

Item (response] functions are powers of each other.... The model

*This work was supported in part by contract H00014-83-C-0457,

project designation NR 150-520 between the Office of Naval Researchand Educational Testing Service. Reproduction in whole or in partIs permitted for any purpose of the United States Government.

Z. ".' -Z Z %.CZZz

F'".... . . . . .

Item Response Functions C

4

rules out normal ogives as well as logistic functions as possible

item [response] functions.*.60C.

more, recently Yen (1964) discussed certain difficulties

encountered In vertical equating usIng a logistic model. She

"hypothesized that [these difficulties I occur because the items

Increase In complexity as they Increase In difficulty." Such

problems might be avoided If the model P(O) satisfying (1) were

used.

In Kristof' a study,* each item is characterized by a single

item parameter. Here, Kristof's result Is generalized, using a

different approach, so as to allow Items to differ in each of two

paramters.

Necessary Condition for Solving the Functional Equation-S*A

Suppose the Item response function for any itemn has the form

P(a*,b 59 6) : it is a function of 0 characterized by two iLemn

parameters, &* and b* . Denote the values of a* and b* for

iteml1by a and b ; for Item 2, by A and 3; for Ite 3 by

a and p*Then (1) becomes

* P(a,b,O)?(A,3,9) a P(*,O,S) .(2)

If such an idenity isto hold for all 0, a, b A, 3,

a ,and pit is necessary that the Item paramters a and

..........

Z!g tZ-' Y;

Item Rsponse Functions

5

be functions of a , b A A, and B (item parmeters by

definition do not depend on 0 ):

a Z (a,bA,B) and 0 0(a~b,AB) .(3)

Define L(a*,b*,O) log P(a*.b*,O) .From (2) .

L(a,b,8) + L(A,BO)B L(a,0,8) . (4)

This is a functional equation In five variables and three unknown

functions, L( a ( ), and 0()

Take the derivative of this with respect to a ,to b ,and

to A:

La(a,b,6) L0 LO,)~ (a L 0(SAP,)0a ,(5)

Lb(a,b,O) Lo,.ab+ LO(CL.S0)0

L A(A,D,O) L(,P,8)A + JP(G,O,9)OA

where each superscript denotes a derivative: for example,

a a a(ab,A,B)/Oa . Continuity and differentiability of

functions are assumed as needed here and in the following derivations.

Given any fixed set of values of the three variables a , 0

and 0 , both L(,0,0) and LO(m,0,6) are fixed. Equations (5)

are then three nonhomogeneous linear equations relating L

I t Ct W,4& l%% Z " -'Z e Z Z Z

Item Response Functions

6

and LO . These three equations will be Inconsistent unless the

augmented atrix

[ (a.b,A,) b ,(a.b.A) Le(ab,6)

b bba (a,b,A,B) 0 (abA,B) Lb(a.b,O)

LA(abAB) 0 A(abA,3) LA(ab.e)

Is siglar. Theefore the corresponding determinant mset vanish:

€',S'- .bSa)LA +. .A)La (b+ ,. _ .A)Lb a o

Reurite this

b,,.b.AB)e a f(a.b.A,2)L. 4 s(+.b&.a5, B)L . (6)

This remalt esesiUisbs the elanatm L nd LO fram

Immediate esieideratLon.

Given any fixed set of values of a, b ,d , both

L8 sad Lb are fixed. ition (6) amproooe a linear relatiouship

betm L and Lb . This relationship mat held as (A,B)

tas" em different values (A1,i), (A2,32). (A3.33). .. . Thus

(6) repreems a (Infinite) set of linar W equatlons

rQlt ng La sd Lb .Tee equatin will be inensistent, if

the 6sa ted istriz,

-? :. - -. . :;,¢- ;;%%.%% %- .;% , %'%.. V', . :..A%.. . %

Item Response Functions

7

fa,hAA0A ) 'aafh.At gy h(&,b.AI 3dLA, 1.0)

f(a,,,A 2 12 ) g(a,b,,A2 '3) h(a,b, A2 , 2 )LA( , 2 0e)

A~ ~f(a'b,%,913) S(a,b,A,.3) hlAb,%,13 )LA(A 3 , 13, e )

Is of rank 3. Thus (using an obvious notation) the determinant

(192 A fg) 39~ + (f3gl flg 3)YiL + "f293 -f 39 2 )hi1t1 0

(7) a..

This result accomplishes the elimination of La and Lb from

Imediate consideration.

Given any fed set ofvaluesof a, b, AI ,B1 A2 2 .

2 A3 , and 13 , the f '. g 'a. nd h'a in (7) are fied.

Equation (7) must still hold for S - e19029e3,..* . Thus (7)

represents an (infinite) set of homogeneous linear equations in the

three fixed quantities (f192 - f2gl)h3 , (f391 - f 1 93 )h 2 , end

(f 2 S3 - f3 A2)hl • Such equations can be consistent only if the . *

matrix

i"

* a

Xtem Response Functions ,"

8

A A AL (Al 13 1 -01 ) L (A2 '3 2 101 ) L (A3 953 901 )

A LAL (A 9. 1 , 62) L (A2 ,2 9a 2 ) LA(A 3 , 0 2 )

LA( A ,e 3) LA(% A2 ,e 3) LA(A 3 B,3e 3) :'

Is of less than full rank. If the equations are consistent, the ".

third row of the matrix must be linearly dependent on the first to ,"

A AA

rove: in other words L A(A,3,0 3 ) ust be a linear function of

LL(A,B, 2 ) and LL((,B, .

This conclusion is a contradiction, since 3 is arbitrary and

cannot be expressed as a function of 0 1 and 0 2 * Thus the prmise

ol tfthat the equations are 'consistent' must be false. This proves that

then varies, (7) represents a set of 'inconsistent' equations,

": according to cossion temtnology. This does not masn, howver thn

(7) is nelrd: the 'inconsistent' boogeneou linear equations

will be satsfied provided

(fig - ~lh fll-f )h2 -(f 3 -f2)hl- 0 . (8) '..

12 YI ' 31 f9 23 f9

This proves that If d afferentiable functions satisfying (4) exist e

4--

then the f og ,and h must Tstsf ( n)o According to (8) and

(7), either t ism e ni.5,

%.k1% - k~J*' . (f."'g" .- ,.°' f%. g1 )h . %3 * (f g1 - f % %)h (fg %- f g2" • 0" *%. (8)- e ' '* ,

Item Response Functions

9

(Case 1) h(a,b,A,B) 3 0 ,or

(Case 2) h(ab,A,B) is nonzero but g(a,b,A,B) I0 and/or

f(a,b,AB) 3 0 , or

(Case 3) f/g Is independent of A,B:

f(a,b,A,B,) -f(a,b, A23B2) F(ab

gj(a,b,A 1 9B1 g(a~b.A, 2 I

where F (a,b) denotes a (more or less) arbitrary function

of a and b only. ~ '

If Case 1 were to hold, then, from the definition of h in

(6)v

30

But for fixed A ,B ,this is the Jacobian of the transformation

a I (a,b) , 0 0(a,b) . When the Jacobian Is zero, a and 0

do not vary independently. In the present problem, this would

mean that for fixed A , B , LUa ,B ,) is only a one-parameter

family, and thus that P(a ,B ,) is only a one-parameter family

of Item response functions. This is contrary to the original

requirements, so Case 1 In Inappropriate.

Q9.

Item Response Functions

10

Case 2 will be treated later. The remaining possibility is

given by (9). Substitute (9) into (6), obtaining

h(a,b,AB)LA(A,B,O) S g(a,b,A,B)[F (a,b)La(a,b,0 )

+ Lb(a,b,6) .

Denote the quantity in brackets by F2(a,b,O) . Take the2V

logarithm of the absolute value of both sides:

logjhj + IA-- log gl f2 (10)

where IA- logiLA, and f2 logIF2 1 " Differentiate (10) on e :A ( . 2 21"

1 A,B,O) -- (a,b,8) . (11) "'-

Now the left side of (11) is not a function of a or b , the

right side is not a function of A or B * It follows that each

side must be independent of all four variables a , b , A , B .

Thus -S..

IA ,Be) (12)

• " ',,.a (sore or less) arbitrary function of e only. .

-f-. .

16%

-. 4,.

Item Response Functions

Integrate (12) on 6 :

A (A,BO) 02(e) + F3 (A,B)

where *2(6) f 1 (O)de and F3 (AB) is the 'constant of

integration.' Exponentiate, obtaining

LA(A,B,8) = xp[ (O) + F (A-B)-

L(A,B, ) - 4 (A,B)*4e )

where F4 and *3 are (more or less) arbitrary nonnegative functions.

Integrate on A to find, finally

L(A,B,O) BF 5(AB)*3(6) + Gl(Be) , (13)

where F5 (A,B) S fF4 (A,B) dA and GI(B,e) is the constant of

integration, both (more or less) arbitrary functions.

The only remaining case, Case 2, also leads to (13), as will now

be shown. If both g(a,b,A,B) and g(a,b,A,B) were zero in Case 2,

(6) would become

h(a,b,A,B)LA(A,B,e) = 0• *

- - • -y . a. .... 9-.- .- T -

Item Response Functions

12

Since h( ) is nonzero in Case 2, this would imply LA - 0 ° But

LA 0 means that the log of the item repsonse function does not

vary with A , a special limiting situation of no general interest

here. The only interesting alternatives under Case 2 is that

either f 0 or else g= 0 , but not both.

When g 0, h* 0 ,and f* 0, then (16) becomes

hLA fLa

Take the logarithm of the absolute value of both sides and then

differentiate on 8 to find

e £e i

A- a

This is equivalent to (11). Thus Case 2 with g - 0 also leads

to (13).

The remaining possibility, that f 0 , hO , and g 0p.. I

(in Case 2) also leads to (13), except that a and b , also A and

B, and also a and 0 are interchanged. Since the original problem-. %

is invariant under this interchange, (13) will still apply, given an

appropriate initial choice of parameter assignment. Thus with

suitable parameter assignment, (13) determines a specific form of

( , ) that is necessary (but not yet sufficient) for satisfying (4).p.'-..

_X~~~~ fce.r,1.

* Item Response Functions .

13

Necessary and Sufficient Condition for a Solution

Using (13), dropping numerical subscripts and rearranging, (4)

can nov be written

[F (a,b) + F (A,B) -F(,)J() 5 G1(A,e) - G1(b,e) - GI(B,O)

(14)

No,*(e) is not identically zero, since by (13) this would make

make L(A,B ,6) and hence L( ,0 ,0) independent of A *Divide by

(14) by 43(6):

F,(a~b) +- 15 (A,B) -FS(aO) IG 2 (0,8) -G 2 (b,e) - G2 (B,O)

(15)

where G2( ) 1( )/4 3 (e) .Differentiate on e

G6 ( 8)=(b,O) + G2-(BO (1)

Since the right side is independent of a and A , (a,b,A,B) must

be also. Hereafter the notation O(b,B) will be used.

Differentiate (16) on b to find:

%2 '' 2

10

I.I

Itm Response Functions

14

Take the logarithim of the absolute value of both sides:

1elogb + ge(5,e) - £O(b.0)

here 0b - *G and go likewise. Differentiate on

6

Repeat the foregoing operations on (15), switching the roles

of b and B to find

8 0go ,o() - gB(B,0)

Elaimnate g0 from the last two equations, obtaining

o ego %b(b,O) S93 (3,0)

Since the left side i not a function of B and the right side

is not a function of b , it is seen that each side is a function

of 0 only; so

Sb

Ste . . . . . e . - e • . .. .

Item Response Functions

15

say*

Integrate this on 60

S(b,) S + k (b)

wehere *2 (0) S * 1i(O)dO and k1 (b) is the constant of

integration. Exponent late:

ObjG2 (b,O)I as expL)*3() +k

Integrate on b and then on 6

G2-(b,O) k3 (b)* 3 (8) + I)

G2 (b,6) ik 3 (b)* 4 (e) + X2 (e) + k4 (b)

Thus, finally, by the relation of G2toG 1 ,

Itm Response Functions

'4o

16

G (bO) a k3 (b)* 5 (e) + k4 (b)# 3 () + Y1) (17)

From (13) and (17),

L(ABO) - 51A B)#3(9) + k3(3)*5() + k4()#31e) + *6(6)

a F6 1A)# 4 (e) + k3 (3)*s(e) + Y1)

From this and (4)

(F6 (ab) + F6 (A,B)L* 4 (e) + [k 3 (b) + k3 (B)]* 5 (0)

- F6 1a&)4 4 (e) + k3 ()* 5 (e) - Y6) •

This shows that *6(6) Is a linear function of 44 and 5(6) :

YO(e) S %4 () + K*5(e)

where C and K are constants (since 6) does not depend on

a , b , A , B ). Substitute this into each of the two preceding

equations, replace 41 ) by *(0), 5(0) by 1(6), and

k ( ) by G( ) , dropping numerical subscripts to obtain finally3

.

-. Item Response Functions

17

L(A,B,8) *F(A,3)4(O) + G(B)Y(O) *(18)

[F(a,b) + F(A,B)]#(S) + [G(b) + G(3)IY(O)

F(*,O)#O) + G(O)Y(9) .(19)

For equality to bold in (19), it is necessary that O(bj)

G ([G(b) + G(B)I where G71( )Is the Inverse function of G()

Also that

*(a,b,A,B) Fb)[F(a~b) + UT(A,I)J

where -1 Is the Inverse function defined for each given b

by F1 IF(a b)I a a

The solution to (2) is found by exponentiating (18):

P(A,B,O) 3 .zp[F(A,B)9(O) + G(B)Y(6)j

3 [f(A,B)j (0)[g(B)j"(0)

.5. UL,(,)F(A) 1 ()G(B) ,(20)

where 0(6) log #(8) , (8) Plog *(6) ,F(A,B) Ulog f(A,B),

and G(B) a log g(B)

. kY -

Item Response Functions

18

The Conjunctive Item Rsponse Function

A reparemsteriuation of the IRT model will simplify (20) for

present purposes. Define new Item parameters by b' s G(B) *

a' a F(A,B) . The general conjunctive Item response function

(20) is om simply

P(a',b',O) i [*(e)J('[#(e)j (21)

Without loss of generality, since #(6) and #(O) remain

to be chosen, take a' ) 0 and b' ) 0 . To be an Item,

response function (lF), it is necessary that P(a',b',e) be a

monotonic increasing function of 6 and that P(a' ,b' ,-..) - 0 ,

P(a',b',e) - 1 * If it is possible to have a' + 0 , then *(0)

must satisfy corresponding conditions. If it Is possible to have

b* + 0 , then #(0) must satisfy corresponding condition.

It will be assumed hereafter that both #(9) and +(0) satisfy

such conditions.

For convenience, the primes In (21) will now be dropped. If

#(e) # *(8) , it would follow that #(0) * #(8) , since otherwise

*(m) u *(.) . I could not hold. If #(8) s #(0) , then

CCbP(a,b,e) . [+(e)])b . In this case, the reparameterization

a" - a + b would reduce P( ) to a one-parameter family. Thus

it is essential that #(0) and #(8) should not be proportional.

r.q

7_ I.4 4'o

Item Response Functions

19

A plausible moel is obtailed by choosing *(0) and #(0)

to be sefamiUar two-parMIter IM,. If the t-parmter

logistic fmection is gmea, for exampl.e, the IRY for Item I Is

-- b?(a:1 ,b1 ,O) (1 + ')(I + e4 (22

where a and pi are arbitrary constants. These constants mst

be the sam for all Items In a given test, buat they can vary

from one test to-another. ideally a and p should be constant9 ,

f or all Items of a single type: they are test or item-type

parameters, not Item parameters.*i~

Up to this point, the argument has dealt With the probability

that a given examines knows the correct aswr to an item or to

a component part of an iten. If the final Item Is presented in

multiple-choice form, however, the examine amy answer correctly

simply by random gueening. To deal with this situation, a

'guessing' paramter ci can plausibly he Introduced into (22),

so that nom

P(&Iqbi~qci,6) C a ci)(l ~"-' ~ + e*o

(23) '5

t5 ;'A

Item Response Functions

20

This IRF does not satisfy the functional equation (2).

Nevertheless, (23) uay be quite appropriate; for example, when

guessing does not contribute to the probability that an examinee

knows the correct answer to the component parts of the Item.

The Disjunctive Item Response Function

The original reasoning dealt with situations where an

examinee would know the correct answer to the actual item only if

he knew the correct answer to two separate subproblems In the

actual item. One can also Imagine situations where the exainee .

will know the answer to the actual item whenever he knows the

answer to either one of two separate subproblems: In other

words, there are two independent routes to knowing the answer

to the actual item. This is the disjunctive case.

The corresponding functional equation is the same as (2)

except that (ab,e) a - P(a,b,O) Is substituted for

P(a,b,8)

Q(ab,O)Q(A,B,O) 5 Q(,,0) • (24)

The corresponding in Is thus

a biQ(ab,O) 11 [1 - (8)] , (25)

. Nor;

P:.

S!

Item Response Functions

21

where #(O) and #(e) satisfy the sae conditions as before. In

the logistic case with guessing,

Q(ai,bici.) 0 1 - ci)1 + eoe+)-0 + a..-

(26)

Practical Implementation and Conclusion

The conjunctive (disjunctive) IRa's have one convenient

feature: When there is no guessing ( c I 0) , the probability of

answering all of a set of a items correctly (incorrectly) has

a such simpler mathematical form than is usually the case. For

conjunctive items, -

Prob(u1 - 1, u2 - ',...,u3 - 1) - i[,( 8 ) b .

(27) .

The parameters aI and bi of conjunctive or disjunctive

Items are not at all readily Interpreted in terms of itemdifficulty and Item discriminating power. It is hard to find even

a complicated function, let alone a simple function, of aI and

and bi that can be comfortably interpreted as a satisfactory

measure of item difficulty or of Item discriminating power.

,P 0

*e

::_",....,,.

Item Response Functions

22

Figure 1 shows six plots of IRF's from (22) illustrating

various degrees of skewness. One can, of course, obtain a pure

logistic curve from (22) by setting either ai or bi equal to

zero. It is also possible to obtain double ogives with three

points of inflexion (not Illustrated here).

A new computer program has been written for simultaneous

eaximutm likelihood estimation of the item parameters a1 and bi

the test parameters a and V , and the ability parameters 8 for

the conjunctive model (23). The new program uses fixed cI

values determined in advance by a run of the data on the computer

program LOGIST (Vingersky, 1983). Although the estimates converge

in any one computer run from a given set of trial values, the result

is apparently not yet useful because from different sets of trial

values the computer reaches different local maxima corresponding

to different assignments of zero values to ai or to bI for

different subsets of items.

The fit of the model to the responses of a group of examinees

could probably be such improved by substituting (26) for (23)

whenever an item Is estimated to have a zero value of a1 or bi

thus assuming that such items are disjunctive items. Subsequent

iterations of the estimation process might then lead to a

.. 1*

.1.

Item Response Functions

23

0.6-0.

4..

1.0 .

6.4- 5.4

-I 0.6 i A

NL Fiur 1. I Illsrtv cojntv ite repos funct I I I ions

wi.h c5(2.*

A.

V" Item Response Functions

24

reassignment of some of these items to the conjunctive model while

other items might be transferred to the disjunctive model.

Ultimately, such a process might successfully classify all items as

conjunctive or disjunctive in such a way as to find a global

maximum of the likelihood function and an optimal fit of the models

to the data. It is not at all clear, however, how much computer

time such a process might require.

It is possible that the conjunctive and disjunctive models,

based as they are on relevant psychological considerations, may

provide a better fit to real data than the usual logistic or normal

ogive models.

5, A, S I* AO '.

4%.o

1'oo

4.

Item Response Functions

References

Kristof, U. (1968). On the varallelization of trace lines for a

certain test model (11-68-56). Princeton, NJ: Educational

*Testing Service.

Wingeraky, M. S. (1983). LOGIST: A program for computing maximum

likelihood procedures for logiatic test models. In R. K.

Hambleton, (Ed.), Applications of item response theory.

Vancouver: Educational Research Institute of British

Columbia.

Yen, W. M. (To be published). Increasing item complexity: A

posilecause of scale shikg for unidlisensional item

response theory. Monterey, CA: CTB/McGraw 1H111.

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