Prime Minister Narendra Modi seems invincible today. Most people think he would win a second term in 2019 general elections. He is popular. He has convinced people that he is working for the nation. He has cracked down on corruption. He wields a strong control over the government and the party. And his winning streak continues even after half his term as the PM when most governments turn defensive.But there is one challenge that he might not be able to defeat in 2019. Chief Economic Adviser Arvind Subramanian has pointed it out in the second edition of the Economic Survey 2016-17 —the lack of reliable jobs data."The debate on the measurement issues on employment and unemployment estimates has been ongoing for some time. The lack of reliable estimates on employment in recent years has impeded its measurement and thereby the Government faces challenges in adopting appropriate policy interventions," Subramanian writes.Employment situation seems to have worsened during Modi's term. According to various estimates, fewer jobs are being created than people entering the job market. However, many in the government think a large number of people are turning entrepreneurs. The government has launched several schemes—from skill enhancement to business loans—to promote entrepreneurship. But by most accounts, unemployment has become a big issue in India today.In the absence of reliable jobs data, the Modi government may not be able to take any more steps to create jobs. The government can remain blissfully unaware of the jobs crisis which can blow in its face in the 2019 general elections. Since PM Modi relies heavily on young voters, the jobs crisis and lack of adequate measures by the government can erode his most crucial voter base.The survey lists a dozen existing government sources of jobs data but also points out their limitations: "Partial coverage, inadequate sample size, low frequency, long time lags, double counting, conceptual differences and definitional issues, rarely used for the purpose of employment estimation, etc.""We don’t really have good reliable consistent series of data to be able to say with some degree of confidence that this went up or this came down. I am hopeful that once we really, improve our employment data, we will be able to do those things much better," Subramanian has said in an interview to ET.The government is carrying out a new employment survey which is unprecedented in its reach and depth. More than 100,000 sample households across 7,500 villages and 5,000 urban blocks will be questioned every quarter to generate closer-to-real-time information, creating seven quarters worth of credible and comparable quarterly and annual labor data for the first time. The first tranche of data is expected to be revealed in December next year, according to a Bloomberg report. The government has trained more than 700 researchers to feed data directly on to tablets instead of the usual paper questionnaires.Creating more jobs was Modi's important poll promise. In his next election, he will be answerable on that count. But if the government neither knows the extent of unemployment nor takes adequate steps to create more jobs, Modi will not be invincible.