Evaluation of the Development Potential of Hainan Free Trade Port Industrial Chain Driven by the Internet of Things Distribution Mechanism

Table 2

Verification of robustness by comparing the single-difference method.

	Single-difference method	Double-difference method
Secondary industry as a share of GDP	0.89	0.93
Annual natural population growth rate	0.04	0.06
City/region individual fixed effects	Control	Control
Fixed asset investment	0.03	0.05
Whether to establish a bonded zone or a free trade zone	Yes	Yes
Expenditures in the budget	2.3%	2.4%