Table 2 Results of measurement properties related to: validity

Structural validity (Factor analysis)

Do the Dutch MFPDI and the NorStOP MFPDI consist of the same factor structure (sub-scales) as the original?

ECA

NL T0

Factors found:

A slightly different factor structure fitted better in both data sets than the previously reported factor structure.

CFA

n = 205

Functional construct: 1-9

UK NorStOP

Pain construct: 10, 14-17

n = 365

Perception construct: 11-13

Previously reported factor structure:

The previously reported factor structure fitted acceptable in the UK dataset, but not in the Dutch dataset.

RMSEA NL: 0.07 UK:0.05

CFI NL: 0.94 UK: 0.98

TLI NL: 0.93 UK: 0.98

Factor structure found in this study:

RMSEA NL: 0.06 UK:0.04

CFI NL: 0.96 UK: 0.99

TLI NL: 0.96 UK: 0.99

Cross cultural validity

Assuming a similar ‘true value’ for foot related disability, does the Dutch population has the same probability of endorsing a certain response option on the items of the MFPDI as compared to the UK population?

DIF analysis using ordinal regression analyses.

NL T0

Foot function sub-scales: no DIF.

Assuming a similar ‘true value’ for foot related disability, the Dutch population has a higher probability of endorsing the response option "none of the time’" or "on some days" on item 17 as compared to the UK population

n = 205

Foot pain sub-scale:

UK NorStOP

Item 17 has DIF;

n = 365

R² = 0.048

Theta for transition score 0 to 1:

NL = -1.38, UK = -0.29

Construct validity (hypotheses testing)

Does the MFPDI relate to other instruments as expected, based on the study of Garrow et al.[7]

Pearsson Correlation*

Comparator instruments:

Pearsson Correlations:

Construct validity is accepted; all hypotheses were confirmed.

Testing 7 a priori defined hypotheses:

1. Correlation MFPDI-f and FFI-f (R > 0.5).

NL T0

1. R 0.66 (p < 0.000)*

2. Correlation MFPDI-f and SF12-phys (R > 0.3).

n = 205

2. R 0.31 (p < 0.000)*

3. R hypotheses 1 > R hypotheses 2

3. R 0.66 > 0.0.31*

4. Correlation MFPDI-p and FFI-p (R > 0.5)

4. R 0.60 (p < 0.000)*

5. Correlation MFPDI-p and pain NRS (R > 0.5)

5. R 0.53 (p < 0.000)*

6. R MFPDI-f - SF-12 phys > R MFPDI-f - SF12 ment

6. R 0.31 > R 0.14 (p = 0.045)*

7. R MFPDI-f - SF-12 phys > R MFPDI-p - SF-12 phys

7. R 0.31 > R 0.22 (p = 0.002)*

*A priori defined hypothesis confirmed