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The DIF-Free-Then-DIF Strategy for the Assessment of Differential Item Functioning

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Establish common metric for DIF assessment

• Equal-mean-difficulty (EMD)• All-other-item (AOI)• Constant-item (CI)

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Equal-mean-difficulty (EMD)• the mean item difficulties of the test for the two groups are

constrained to be equal.• Software: ConQuest, BILOG-MG PARSCALE• EMD is true when (a) no DIF item, or (b) multiple DIF items but

some favor the reference group and others favor focal group, and the DIF amount are identical. i.e., ASA 0. When ASA >>0, EMD ≒is severely violated.

• • When applied EMD method, overall, the DIF amount in a test is

forced to be balanced between groups. Therefore, half items will be classified as favoring the reference group while another half as favoring the focal group. i.e., DTF=0.

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All-other-item (AOI)

• all but the studied item in the test are treated as DIF-free and served as a matching variable

• • Software: Winsteps, IRTLRDIF, CFA-based DIF

detection• • AOI is true when (a) the test is perfect, i.e., no

any DIF item, or (b) the studied item is the only DIF item in the test

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Constant-item (CI)• select a subset of items to serve as anchor to establish a

common metric for DIF assessment• • The CI method yields appropriate DIF assessment even when

the test contains 40% DIF items. In addition, the longer the anchors, the higher the power rate of detecting DIF. Moreover, four anchors are generally enough to yield a high power rate.

• • The superiority of the CI method is verified in the CFA-based

DIF assessment (MIMIC method) and IRT-based DIF detection technique.

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Scale purification procedure

• Three major steps:• calibrate item parameters separately for each group,

compute the linking coefficients (a slope and intercept) to place the reference & focal groups on a common metric and assess each item for DIF.

• re-link group matric using only those items not identified as having DIF in the previous step, and assess each item for DIF.

• repeat step b until the same items are identified as having DIF in consecutive iterations.

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• DIF assessment method with scale purification procedure provide a substantial improvement over EMD and AOI method. But it will come worse when they are high percentage DIF items (>20%, constant DIF)

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Implementing the DFTD Strategy

• Woods’ Rank-based (RB) method to select DIF-free items

• assess item 1 for DIF using all other items as anchors, and repeat this step until the last item is assessed.

• compute the ratio of the likelihood ratio statistic (LR) to the number of free parameters for each item (df), i.e., LR/df.

• rank order the items based on the ratio LR/df.• designated the desired numbers of items (generally 4

items) that have the smallest ratios.

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• Findings: • RB method has very high rate of accuracy in selecting

clean anchors in large DIF items and large sample size. unknown on small sample size

• the rate of accuracy decreased when the percentage of DIF items increased. need purification procedure

• the designated anchors yielded more accurate DIF assessment than the AOI method in terms of Type I error rate and power rates. support DFTD strategy.

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• Rank-based scale purification method (RB-S): to add a scale purification procedure to the RB method.

• assess item 1 for DIF using all other items as anchors and repeat this step until the last item is assessed.

• remove those items that are identified as having DIF from the anchors and re-assess all items for DIF using the new anchors;

• repeat step b until the same set of items are identified as having DIF in two consecutive iterations

• designed the desired numbers of items that have the smallest ratios (LR/df).

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Simulation Study one

• Design: (1) method: RB vs RB-S; (2) impact: r ~ N(0,1); f ~ N(0,1) or N(-1,1) (3) sample size: 250/250, 500/500, 1000/1000 (4) test length: 20, 40 (5) DIF items Percentage: 10%, 20%, 30%, 40% (6) DIF patterns: constant and balanced

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• 100 replications• Data gen model: 2PL logistic model• DIF magnitude: ~ N (0.6, 0.01). ASA• Software: IRTLRDIF• Dependent Variable: Rate of accuracy, Type I

error rate, Power,

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Results• Rate of Accuracy:• (a) Both of RB & RB-S methods yields a rate of accuracy higher

than the chance level;• (b) the larger the sample size and the longer the test, the higher

the rate of accuracy;• (c) RB& RB-S methods yield similar rates of accuracy when ASA

was very close to zero (e.g., under the balanced DIF pattern).• (d) RB-S method yield a rate of accuracy higher than that of RB

method when the ASA was far away from zero (e.g., high percentage of DIF under the constant pattern) support DFTD strategy.

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Relationship between the ASA and the rate of accuracy for the RB and RB-S methods for large (R1000/F1000; 1c) in study 1

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Result

• Type I error & power rate• (a) when ASA was close to zero, RB & RB-S

method yield a well-controlled Type I error as well as high power rate.

• (b) when ASA was far away from zero, RB method yield an inflated Type I error rate and a deflated power rate whereas RB-S method maintain a good control over Type I error rate and power rate.

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Simulation Study two• Purpose: compare RB-S method and AOI, AOI-S, PA

• RB-S method: use RB-S method to select four anchor and then assess the other items for DIF with the designated anchors (purification procedure, DFTD).

• AOI method: all other items were treated as anchors when assessing the studied item for DIF (no purification, no DFTD)

• AOI-S method: the AOI method with scale purification procedure (purification procedure, no DFTD)

• PA: pure anchor method, in which four clean anchors were selected by design and then the other items were assessed for DIF.

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• Design: (1) method: RB vs RB-S; (2) impact: r ~ N(0,1); f ~ N(0,1) or N(-1,1) (3) DIF item percentage: 10%, 20%, 30%, 40% (4) DIF pattern: constant, balanced (5) sample size: 500/500 (4) test length: 20 items• Dependent variable: Type I, power

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Result

• (a) When ASA was very close to zero, all methods yield a well controlled Type I error rate and the AOI and AOI-S method yield a power rate higher than that yielded by RB-S and PA methods.

• (b) When ASA was large, only RB-S and PA method maintain control over Type I error rate and power rate, whereas AOI method suffered from the large ASA seriously, followed by AOI-S method.

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Relationship between the ASA and the mean Type I error rate for the AOI, AOI-S, PA, and RB-S methods in study 2

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Conclusions and Discussions

• Limitation of IRTLRDIF in implement CI method.• Some other methods to select DIF-free item.

e.g., the iterative constant-item method (Shih & Wang, 2009);

• Nonuniform DIF;• Rash model, 3PL, GPCM,…• Implement DFTD to IRT/nonIRT based DIF assess

method (MH, LR)

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ASA

• Average Signed Area• • SA is the signed area measured between the

two items’ characteristic curves for two groups on item .

• (under Rasch, 2PL) where and are the difficulties of item of the focal and reference group, respectively.

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• ASA ≒0: test do not contain DIF item or when tests contain multiple DIF items but their effects are cancelled out between groups (balanced pattern)

• ASA > 0: the test as a whole favors the reference group;

• ASA < 0: the test as a whole favors the focal group;