This study is the first to examine the predictive value of two applications of the iPhone, which have the capability to measure cervical ROM using the CROM as the accepted gold standard. Although a few studies were already done on the validity of a digital device for the measurement of cervical ROM, no previous study was done on the digital inclinometer and/or the compass of the iPhone for the measurement of cervical ROM.
Reliability estimates are very important psychometric properties since before an instrument can be considered valid, it needs to be reliable. The findings of this study showed that when the cervical ROM is measured with the iPhone by the same examiner (intra-rater reliability), similar results can be expected from one session to the next. It is possible to compare the current results of intra-rater reliability with digitals inclinometers such as Electronic Digital goniometer (EDI-320) for the measurement of active neck movement. For instance, in the sagittal plane, we found good reliability for flexion (ICC = 0,78; 95% CI: 0,58-0,89) and extension (ICC = 0,84; 95% CI: 0,69-0,92). Tousignant et al.  also reported good reliability for flexion (ICC = 0.77; 95% CI: 0,62-0,87) and extension (ICC = 0,83; 95% CI: 0,63-0,92) using the EDI-320. However, this device is not on the market anymore. In comparison, the reliability coefficients found in our study were slightly higher.
For the frontal plane, we found good intra-rater reliability (ICCs = 0.77-0.78; 95% CI: 0,56-0,89), while Prushansky et al.  found similar results with a digital inclinometer (ICCs = 0.82-0.90; 95% CI: 0,61-0,95).
Finally, in the transverse axis, we found moderate to good intra-rater reliability (ICCs = 0.66-0.74; 95% CI: 0,39-0,87), while Prushansky et al.  observed higher ICCs (ICCs = 0.84-0.92; 95% CI: 0,68-0,96). This might be explained by the fact that they took their measurements with the inclinometer while the subjects were in supine position, whereas we used the compass rather than the inclinometer of the iPhone. Since the compass is not influenced by gravity, but rather by orientation of the iPhones, it has more potential source of error than the inclinometer, which could have easily influenced the intra and inter-rater ICCs. Furthermore, the magnetometer which serves as the hardware for the compass application is more sensible of the presence of electro-magnetic fields which is another factor that could have contribute to the lower ICCs for the measurements of neck rotation.
When the ROM measured by two independent examiners were compared, our ICCs were moderate for movements in the sagittal plane (ICCs = 0.48-0,49; 95% CI: 0,14-0,72) and in the frontal plan (ICCs = 0.40-0,54; 95% CI: 0,04-0,75). When we look closely at our results, we found that examiner 2, who used an iPhone generation 3GS, always had higher ROM measures than examiner 1, who used an iPhone generation 4. Considering that Apple uses an LIS302DL accelerometer for both iPhones 4 and 3GS and the two different generations of iPhone had the same operating system (iOS 4), factors related to the positioning of the iPhone might explain this observation. We also found poor correlation in transverse plan (ICC = 0.07-0,09; 95% CI: -0,30-0,44), which again might be explained by the presence of electro-magnetic fields that could influence the measure. On the other hand, it could also be attributed to the examiner since examiner 2 showed lower intra-rater reliability.
Cervical ROM measured with the iPhone presented comparable results (moderate to good validity) when compared to the ROM measured with the CROM for all cervical movements, except for the movement of left rotation (ICC = 0.43). On the basis of this relation, the validity of the iPhone can be considered good for these movements for a same examiner, except for rotation. The poor results observed for the movements of rotation (ICC < 0.60) may partly be explained by the fact that it was measured by an application very sensible to electro-magnetic fields. This can lessen the accuracy of the, measurement. It could also be explained by the movement and/or positioning of the iPhone during the measurement of cervical rotation.
To our knowledge, no study examining the validity of the iPhone for assessing cervical ROM has been published. However, a recent article on the reliability and validity of a relatively inexpensive digital inclinometer reported results that were similar to our findings in sagittal and frontal planes: a good reliability (ICCs = 0,82-0,94) but lower validity (r = 0,62-0,83). Results were different for the rotation movements: a good reliability (ICCs = 0,84-0,92) and poor validity (results not reported). Their better results obtained for the reliability of rotations might be explain by the fact that rotations measured in supine position .
Our results show that measures of extension and right rotation had poor inter-rater reliability and thus mined the validity of this measure. This discrepancy may be attributed to the data collection procedures or the placement of the iPhone on the top of participant’s head. Special efforts were made in this study to minimize this type of error, but we suggest that future measurements of rotation movements might be done with the iPhone on the top of the forehead while the person is lying supine as done by Prushansky .
Strengths and limitations
First, the two examiner’s initial preparation (training) with the CROM represents strength. The assessment of the examiner’s skills showed that they were competent (ICC > 0,65) in the use of the method and the device (E1: ICC = 0,81; 95% 0,56-0,92. E2: ICC = 0,79; 95% 0,52-91) (see Table 1 for ICC reference values). For the validity study, we purposely chose examiner 1 in order to minimize the source of error coming from the examiner.
Second, standardization of the procedures also helped minimize random errors. To achieve this, all participants were stabilized in order to avoid compensation. Also, the research assistant always gave the same instructions before each measurement for all participants and the environment was identical during all the data collection process: same rooms, same orientation or the participants (facing east), same chairs, etc.
Thirdly, measures were taken with the iPhone and the CROM were always taken in the same order. Thus, if the cervical ROM increased with repetitions, the pattern would be the same for all participants and would not influence our results.
Finally, the iPhone measures were always taken before the CROM measure to prevent an information bias. Due to the numerous measurements took with the iPhone, we considered that it would have been impossible for the examiner to remember all the results and influence its readings using the CROM. Therefore, we think that this help minimized an information bias.
This study also had limitations. First, data was collected on a sample of healthy participants, which limits the external validity. Although we tried to minimize bias affecting the internal validity, but the fact that examiner 1 had higher intra-rater ICCs than examiner 2 might partly explain the modest results for the inter-rater reliability.