feat(notes): Add data validity ranking documentation

03-documentation/validity_tool.md

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+# Validity estimators
+
+For a general concept description see ../03-documentation/terms_of_reference-key_terms.md#validity
+
+From Maitrot2017 -> Section 4, Figure 3 and Appendix table notes
+They rank *only* quasi-experimental/experimental
+
+Do we too?
+
+## internal
+
+
+0-5 ranking using study design/method:
+
+OLS=2 (ordinary least squares)
+DID=3 (difference-in-difference)/(-in-difference)
+DM=3 ()
+PSM=3.5 (propensity score matching)
+IV=4 (instrumental variable)
+RD=4.5 (regression discontinuity)
+RCT=5
+
+## external
+
+non-representative surveys=2
+sub-nationally representative surveys=3
+nationally representative surveys=4
+surveys based on census data=5