• From 1957 January to 1975 July, the base year was 1939=100. During this period, no linking factor was adopted. • Initially, up to 1960, there were 12 centres—Thiruvananthapuram, Kollam, Punalur, Alappuzha, Changanacherry, Kottayam, Munnar, Aluva, Ernakulam, Thrissur, Chalakkudy, and Kozhikode. • In 1961, the Cherthala Centre was newly introduced, making the total 13 centres during the 1939=100 period (1957–1975 July). Index marked April onwards. • From 1975 August to 2001 September, the base year was 1970=100, and linking factors were introduced for the respective centres. • During 1970=100 (1975 August–2001 September), Cherthala, Changanacherry, and Aluva centres were deleted, and five new centres—Mundakayam, Palakkad, Malappuram, Meppady, and Kannur—were added, increasing the total to 15 centres. The newly formed five centres did not have linking factors. • From 2001 October to 2015 March, the base year was 1998–99=100, and linking factors for each centre were revised accordingly. • In the 1998–99=100 period (2001 October–2015 March), two more centres—Pathanamthitta and Kasaragod (no linking factor)—were formed, bringing the total to 17 centres. • From 2015 April onwards, the base year is 2011–12=100, and the linking factors are in operation up to May 2025. • For the current 2011–12=100 period (from 2015 April to 2025 May), all 17 centres have linking factors. Note that the centre Munnar is replaced as Idukki in 2015, and Meppady as Wayanad in 2025.