## 4 Result  
### Efficiency Analysis Results  
In this study, the three-year data from 2011 to 2013 of representative wireless carriers in USA--Verizon Wireless, Sprint Nextel, AT & T, and T-Mobile--are set as DMUs. Table 2 below shows the basic statistic of each factor. Each data is collected from the previously mentioned institution, J.D. Power and Novarum, and annual financial reports of each company. According to [PERSON] and [PERSON] (2016), the appropriate number of input factors are limited by DEA while the appropriate number of output factors are limited by the DMU. This is because an increase in the number of input and output factors in the DEA results in an increase in efficient DMUs. This, in turn, makes it difficult to distinguish the inefficient DMUs. [PERSON], et al. (2010) and [PERSON] (2017) claimed that the number of DMUs should be 3 times greater than the sum of the number of input and output factors, and [PERSON], et al. (2018) claimed that the number of DMUs should be greater than the product of the number of input and output factors. In addition, [PERSON] and [PERSON] (2019) suggested that the number of DMUs should be twice as greater than the sum of the number of input and output factors. The number of DMUs and that of the input and output factors used in this study are 12, 2, 2, respectively, which all satisfies the criteria given by [PERSON], et al. (2010), [PERSON] (2017) as well as [PERSON], et al. (2018) and [PERSON] (2019). Thus, the number of DMUs used in this study are appropriate for DEA. The results of DEA verified using bootstrap DEA are listed in Table 3. The results indicate that the most efficient wireless carrier in terms of management is T-mobile in 2011, Verizon Wireless in 2012, and AT & T in 2013, all of which show efficiency scores of 1. Figure 3 below presents a visual representation of how each of the DMUs performs in terms of service quality efficiency and network quality efficiency. A look at Figure 3 suggests that Sprint Nextel \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Factors** & **Mean** & **Sum** & **SD** & **Min** & **Max** & **Unit** \\ \hline Operating Expenses & 66,194.58 & 794,335 & 39,527.32 & 23,424 & 117,505 & million USD \\ Total Asset & 149,987 & 1,799,844 & 11,4957.7 & 9482.93 & 277,787 & million USD \\ Customer Care & 762 & 9144 & 17.68 & 739 & 795 & 1000 points (Likert scale) \\ Purchase Experience & 762.75 & 9153 & 22.47 & 741 & 798 & 1000 points (Likert scale) \\ Revenue & 74,453.25 & 893,439 & 49,769.7 & 19,719 & 128,752 & million USD \\ Operating Income & 8441.17 & 101,294 & 12,626.03 & \(-\)6397 & 31,968 & million USD \\ Download Speed & 3.29 & 39.45 & 2.72 & 0.62 & 9.12 & Mbps (4G) \\ Upload Speed & 1.65 & 19.81 & 1.77 & 0.59 & 5.86 & Mbps (4G) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Descriptive statistics of input and output factors. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**DMU** & \multicolumn{4}{c}{**Bootstrap Efficiency Mean**} \\ \cline{2-5}  & **Management** & **Service Quality** & **Network Quality** \\  & **Efficiency** & **Efficiency** & **Efficiency** \\ \hline Verizon Wireless-2011 & 0.9300 & 0.2422 & 0.1068 & 1.0000 \\ Sprint Nextel-2011 & 0.9196 & 0.7024 & 0.3283 & 0.2524 \\ AT \& T-2011 & 0.8938 & 0.1970 & 0.1274 & 1.0000 \\ T-Mobile-2011 & 1.0000 & 1.0000 & 1.0000 & 0.1601 \\ Verizon Wireless-2012 & 0.9518 & 0.2287 & 0.1183 & 1.0000 \\ Sprint Nextel-2012 & 0.9264 & 0.7045 & 0.3188 & 0.2548 \\ AT \& T-2012 & 0.9112 & 0.2001 & 0.1384 & 1.0000 \\ T-Mobile-2012 & 0.7500 & 0.8945 & 1.0000 & 0.1532 \\ Verizon Wireless-2013 & 1.0000 & 0.2749 & 1.0000 & 1.0000 \\ Sprint Nextel-2013 & 0.9296 & 0.7238 & 0.5194 & 0.2627 \\ AT \& T-2013 & 1.0000 & 0.2493 & 0.7852 & 1.0000 \\ T-Mobile-2013 & 0.8885 & 1.0000 & 1.0000 & 0.1897 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Result of bootstrap DEA. primarily focuses on service quality and is gradually improving its network quality efficiency.