Is Spline Interpolation suitable for Economic Data I have GDP data recorded in quarterly and I wish to interpolate it for monthly data. Is the Spline Interpolation suitable for these type of economic data?

Yes, using spline interpolation for this task is fine. Just to state the obvious though: we will exaggerate the autocorrelation of our new monthly data so it will be crucial to actively account for it. (e.g. by assuming an AR error structure in our residuals, etc.)
Some quick references I came across using exactly this approach:

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*Derivation of Quarterly GDP, Investment Spending, and Government Expenditure Figures from Annual Data: The Case of Pakistan by Rashid & Jehan.

*Cubic Spline Interpolation: A robust method of disaggregating annual data to quarterly series by Ajao et al.

While I have not used, it seems that SAS has some functionality exactly for that in EXPAND Procedure where: "the EXPAND procedure fits cubic spline curves to the nonmissing values of variables to form continuous-time approximations of the input series. Output series are then generated from the spline approximations." This once again suggests spline interpolation is a reasonable approach for such a task.
