The CPCB had monitored several pollutants between April 1 to 14 before the odd-even period and April 15 to 30 during it to arrive at the conclusion.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/decline-vehicular-emission-had-no-impact-pollution-cpcb
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/indian-express-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/central-pollution-control-board-cpcb
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-quality-monitoring
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/national-green-trbunal-ngt
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/pm-25
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/rspm-pm-10
[11] http://admin.indiaenvironmentportal.org.in/category/thesaurus/carbon-monoxide
[12] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects
[13] http://admin.indiaenvironmentportal.org.in/category/thesaurus/indian-institute-technology-iit