Table 1.

Steps for Processing Skyn Data to Identify Drinking Episodes

Step Number	Step name	Description
1	Recoding negative TAC values to zero	All Skyn TAC values that were negative (less than zero) were replaced with zero. This step was recommended by BACtrack Skyn personnel due to the fact that there are no zero values in Skyn dataa and negative TAC values are physiologically implausible.
2	Recoding TAC values with temperature <29 °C to missingb	All Skyn TAC values that occurred when the temperature reading was <29 °C were considered missing data and were recoded as missing. This step was recommended by BACtrack Skyn personnel because the Skyn monitor measures body temperature (°Celsius) as a means to detect non-wear time. Personnel suggested that any temperature reading <29 °C was indicative of participant non-wear.
3	Calculating 15-minute rolling averages for TACc	15-minute rolling averages for TAC (μg/L air) were calculated that included the 7 minutes of data prior to and after a given minute of data. This step was recommended by BACtrack Skyn personnel to smooth out TAC data.
4	Identifying minutes with rolling TAC ≥15 μg/L air	Any row of data with a 15-minute rolling average for TAC ≥15 μg/L air was identified as potentially representing the device being exposed to alcohol, either via the participant’s skin, which would be indicative of a participant consuming alcohol, or via environmental contamination (i.e., hand sanitizer, cologne, alcohol-based products). BACtrack Skyn personnel identified 15 μg/L air as a cut-off TAC value (from the rolling average) that could potentially represent true alcohol exposure. This step was needed due to the lack of zero values in Skyn data, such that a person could have a zero for blood alcohol concentration but still show positive TAC readings from the Skyn device.
5	Bracket individual alcohol exposure episodesd	Individual alcohol exposure episodes were bracketed separately from one another, such that consecutive strings of data with rolling TAC averages ≥15 μg/L air were bracketed into an ‘alcohol exposure episode’ to indicate that the device was exposed to non-negligible amounts of alcohol.
6	Define characteristics of alcohol exposure episodes	Characteristics of the alcohol exposure episodes were defined, including the peak TAC, AUC, episode duration, average ascending TAC slope, and average descending TAC slope. This step was used as a starting point for identifying episode characteristics that were indicative of actual alcohol use (i.e., actual drinking episodes) versus characteristics that were indicative of non-drinking alcohol exposure (i.e., environmental contamination).
7	Calculating characteristics for Skyn alcohol exposure episodes	Descriptive statistics for each of the characteristics defined in Step 6 were calculated for each Skyn alcohol exposure episode. This step was used to identify minimum, maximum, and mean values of episode characteristics (e.g., peak TAC, duration) to indicate values that were likely representative of actual alcohol use versus non-drinking alcohol exposure, such as environmental contamination.
8	Removing Skyn alcohol exposure episodes ≤15 minutes in duratione	Skyn alcohol exposure episodes ≤15 minutes in duration were removed. Episodes ≤15 minutes were considered to be indicative of non-drinking alcohol exposure based on descriptive characteristics calculated during Step 7 and based on the physiological improbability that an individual would metabolize all of the alcohol they consumed rapidly enough to make it undetectable by the Skyn (i.e., rolling average <15 μg/L air) in ≤15 minutes. This step is similar to Roache et al.’s Rules 5 and 6 (2019).
9	Removing Skyn alcohol exposure episodes ≤60 minutes in duration and with a peak TAC ≥400 μg/L airf	Skyn alcohol exposure episodes ≤60 minutes in duration and with a peak TAC ≥ 400 μg/L air were removed. Episodes with these characteristics were considered to be indicative of non-drinking alcohol exposure based on Step 7, as the combination of a high peak TAC with a short episode duration reflected the Skyn being exposed to non-drinking-related alcohol, like hand sanitizer, resulting in a higher peak TAC and shorter duration than would be likely if an individual consumed enough alcohol to reach such a high peak TAC. If an individual consumed that much alcohol, it would likely take longer than 60 minutes to metabolize all of the alcohol rapidly enough to make it undetectable by the Skyn in ≤60 minutes.
10	Removing Skyn alcohol exposure episodes with a rise rate ≥2500g	Skyn alcohol exposure episodes with a rise rate ≥2500 were removed. Episodes with this characteristic were considered to be indicative of non-drinking alcohol exposure based on Step 7, as such large rise rates were considered implausible representations of true drinking, as the TAC increased too quickly over too short of time period for true drinking, and likely only occurred due to environmental contamination, such as exposure to high concentrations of alcohol (i.e., hand sanitizer) at the Skyn sensor. This step is similar to Roache et al.’s Rule 3 for processing SCRAM data (2019).
11	Removing Skyn alcohol exposure episodes ≤60 minutes in duration and with a rise rate ≥1000h	Skyn alcohol exposure episodes ≤60 minutes in duration and with a rise rate ≥1000 were removed. Episodes with these characteristics were considered to be indicative of non-drinking alcohol exposure based on Step 7, as such short episodes combined with such high rise rates were considered implausible representations of true drinking, as the TAC increased too quickly over too short a time period and the episode was too short in duration for an individual to metabolize the amount of alcohol that could account for such a rapid increase in TAC over that short time period of ≤60 minutes.
12	Removing Skyn alcohol exposure episodes with a rise rate = 0i,j	Skyn alcohol exposure episodes with a rise rate = 0 were removed. Episodes with this characteristic were considered to be indicative of non-drinking alcohol exposure based on Step 7, as actual alcohol consumption should be reflected by increases (ascending limb/slope) and decreases (descending limb/slope) in TAC paralleling changes in blood alcohol concentration after alcohol is consumed, metabolized, and eliminated from the body, and a rise rate = 0 reflects no increase in TAC across the episode.
13	Replacing non-zero TAC values that were not part of an alcohol exposure episode with 0 μg/L air	All non-zero TAC values that were not part of an alcohol exposure episode, including negligible non-zero TAC values, as well as TAC values that were originally part of an alcohol exposure episode, but that were later removed based on Steps 8–12, were replaced with 0 μg/L air, as the original non-zero values were deemed to not represent drinking-related alcohol exposure. They were not replaced with missingvalues because the data were not missing perse, given that the Skyn had TAC measures that were taken when the device was being worn (the temperature was ≥29 °C). These values were replaced with 0 so that we could calculate day-level characteristics of alcohol use from the Skyn TAC data with non-drinking related TAC values removed from the day-level data.

^aRaw Skyn data did not include any values equal to zero.

^bDescriptive statistics of TAC readings that occurred when the temperature was <29 °C revealed that all readings were negligible, non-zero values, increasing confidence that the cut-off of 29 °C was appropriate and likely did not inadvertently result in removing drinking episodes from the data.

^cSmoothing of TAC data through the use of rolling or moving averages is an approach that has been used by previous researchers (Rosenberg et al., 2021; Wang et al., 2019).

^dThis first round of bracketing alcohol exposure episodes resulted in identifying 414 episodes across the 11 participants’ 308 study days, with a range of 1–63 episodes per participant.

^e108 episodes violated the rule of being ≤15 minutes in duration.

^f70 episodes violated the rule of being ≤60 minutes in duration and having a peak TAC ≥400 μg/L air.

^g118 episodes violated the rule of having a rise rate ≥2500.

^h165 episodes violated the rule of being ≤60 minutes in duration and having a rise rate ≥1000.

ⁱ21 episodes violated the rule of having a rise rate = 0.

^jAfter removing a total of 260 episodes based on rules outlined in steps 10–14, there were 154 episodes across 10 participants (one participant had zero episodes).