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Tracking billable hours is one of the most tedious aspects of client-based work, as anyone who has had to fill out time sheets can attest. As it happens, I have saved (almost) all of my time sheets since I started working my current job back in April 2009 in Word Document tables. That’s about 79 time sheets.
The source code for the data analysis can be found here.
We can start by plotting weekly billable hours since April 2010, when I started working as a permanent, full-time employee. As shown in Figure 1 below, there is significant variability in my hours from week to week, but the trend appears to suggest a gradual decrease in hours over time. Let’s smooth out the week to week variability by calculating rolling averages.
Figure 1. Weekly Billable Hours
Figure 2. Weekly Billable Hours (Moving Average)
Figure 3 shows a frequency distribution of weekly billable hours along with summary statistics. I bill 43.8 hours per week on average with a standard deviation of 11.8 hours. A quarter of the time, I bill 51 or more hours per week. The busiest week was 79 hours.
Figure 3. Distribution of Weekly Billable Hours
I was also curious to know if the week to week variability in my hours has changed over time. To measure this variability, I calculated the standard deviation of billable hours over rolling four week periods. I then smoothed out the figures by calculated a sixteen week moving average of the four week standard deviations. The result is shown in Figure 4. While it appears as though the variability has decreased over time, the results are distorted by a spike in 2011. Excluding the spike in 2011, the variability is unchanged. My schedule is still generally unpredictable. In fact, the standard deviation over the last four weeks was in the top 15th percentile historically (including the spike in 2011).