# Effects of Wearing a Swimming Cap and Goggles on Phoria and Fusional Vergence
## 2 Materials and Methods
### Measurement of Phoria and Fusional Vergence  
For all study participants, the order in which the measurements were recorded after clinical refraction was distance phoria, near phoria, NFV at distance, PFV at distance, NFV at near, and PFV at near.  
Both phoria and fusional vergence while wearing swimming goggles should be measurable in free space at distance (6 m) and at near (40 cm). To this end, the following conditions were required: a Howell phoria card for distance (3 m) (CDHPC; Cyclopean Design, Heathmont, VIC, Australia) and near (33.3 cm) (CDHPCN; Cyclopean Design) modified to 6 m (1 \(\Delta\), 1 prism dipoter = 6 cm) and 40 cm (1 \(\Delta\) = 0.4 cm). NFV and PFV were measured using handheld Risley rotary prisms (ARPHH30; Bernell, Mishawaka, IN, USA).  
For the Howell phoria card, a 6 \(\Delta\) base-down prism was placed in front of the right eye. In the lower chart image, the numbers on the right indicated by the arrow represent exophora whereas the numbers on the left represent esophoria. Phoria was measured on three consecutive occasions at distance followed by at near.  
The NFV values at distance and near were measured using the handheld Risley prisms. A single letter (0.5 decimal) was used for fixation at distance and at near. The Risley prisms were increased gradually at a speed of 1-2 \(\Delta\)/s of base-in (BI) prism and participants were asked to report when the fixation object appeared to blur. If the participant reported a sustained blurring at a point then this point was considered a blur point. If the participant did not report any blurring but instead reported dipping first, then there was deemed to be no blur point. The amount of prism was increased until the participant reported sustained double vision. The prism power was noted at the break point and the prism power was then gradually decreased. Thereafter, the point at which the patient regained single vision was recorded as the recovery point. Blur points, break points, and recovery points of PFV at distance and near were measured using handheld Risley base-out prisms. Changes were not analyzed because the blur points of NFV and PFV did not appear in many cases [27].  
[PERSON]'s criterion requires the compensating vergence (PFV or base-out for exophoria; NFV or base-in for esophoria) to blur and to be twice the heterophoria. A prism can be prescribed if this criterion is not met. Clinically, the amount of prism that needed to be prescribed to meet [PERSON]'s criterion was determined as two-thirds the phoria minus one-third of the fusional vergence [28]. If there was no blur point, the break point was used instead.  
The coefficient of repeatability was calculated by multiplying the standard deviation of differences between measurements by 1.96 to detect changes in phoria between measurements. Values within and outside the range of the coefficient of repeatability were determined to represent no change and change, respectively. Therefore, the respective criteria for change in phoria at distance and near were 1.10 \(\Delta\) and 1.60 \(\Delta\). Applying the 95% limits of agreement (LOA) in rotary prism fusional vergence [29], the criteria forchanges in fusional vergence were over 2.5 \(\Delta\) for the break and recovery points of NFV at distance; over 4.0 \(\Delta\) for the break point and 2.5 \(\Delta\) for the recovery point of NFV at near; over 5.5 \(\Delta\) for the break point and 5.0 \(\Delta\) for the recovery point of PFV at distance; and over 4.0 \(\Delta\) for the break point and 5.5 \(\Delta\) for the recovery point of PFV at near.