I want to test the correlation of a variable with investment in the national income formula C+I+G+NX. Using IMF data, there are a lot of different investment variables in different formats. What would be the most appropriate variable or variable combination for this? I've also found World Bank Data in %GDP which appears to also be from the IMF. The description from World Bank is "Expressed as a ratio of total investment in current local currency and GDP in current local currency. Investment or gross capital formation is measured by the total value of the gross fixed capital formation and changes in inventories and acquisitions less disposals of valuables for a unit or sector." If this data is viable, then for discussion, does this mean, for example, "New trucks minus sold trucks, domestically"? Or does this include international capital, e.g., my trucks purchased and driven abroad but for income sent home?
You can find data on GDP and its components from IMF at this link.
IMF separates investment into Gross Fixed Capital Formation and Change in Inventories. If you sum these two variables together you will get the $I$ portion of GDP.