diff --git a/mef/report_layer.rst b/mef/report_layer.rst
index 01e9a11..989e2ce 100644
--- a/mef/report_layer.rst
+++ b/mef/report_layer.rst
@@ -87,6 +87,7 @@ The following three categories are so frequent
 that is it worth to normalize the way they are stored into XML files.
 
 - Minimal cut sets (and prime implicants)
+- Importance factors
 - Statistical measures (with moments)
 - Curves
 
@@ -108,6 +109,31 @@ to carry the relevant information.
     :caption: The RNC schema for the XML representation of sums-of-products
     :language: rnc
 
+Importance factors
+------------------
+
+Importance factors are typically computed for basic events
+with the same configurations as for probability analysis.
+The following importance factors are long-established and commonly calculated:
+
+    - Fussel-Vesely Diagnosis Importance Factor (DIF)
+    - Birnbaum Marginal Importance Factor (MIF)
+    - Critical Importance Factor (CIF)
+    - Risk Reduction Worth (RRW)
+    - Risk Achievement Worth (RAW)
+
+The definitions of these factors can be found in PSA/PRA literature.
+
+In addition to the importance factors,
+this report section may contain
+the evaluated probability of an event and
+the number of products it occurs in.
+
+.. literalinclude:: schema/importance_factors.rnc
+    :name: schema_importance_factors
+    :caption: The RNC schema for the XML representation of importance factors
+    :language: rnc
+
 Statistical measures
 --------------------
 
diff --git a/mef/schema/importance_factors.rnc b/mef/schema/importance_factors.rnc
new file mode 100644
index 0000000..7a21bee
--- /dev/null
+++ b/mef/schema/importance_factors.rnc
@@ -0,0 +1,19 @@
+importance =
+  element importance {
+    name?,
+    attribute description { text }?,
+    attribute basic-events { xsd:nonNegativeInteger }?,
+    importance-event*
+  }
+
+importance-event =
+  element basic-event {
+    name,
+    attribute occurrence { xsd:nonNegativeInteger }?,
+    attribute probability { xsd:double }?,
+    attribute DIF { xsd:double }?,
+    attribute MIF { xsd:double }?,
+    attribute CIF { xsd:double }?,
+    attribute RRW { xsd:double }?,
+    attribute RAW { xsd:double }?,
+  }
diff --git a/spelling_wordlist.txt b/spelling_wordlist.txt
index 26c9327..1a31286 100644
--- a/spelling_wordlist.txt
+++ b/spelling_wordlist.txt
@@ -1,12 +1,15 @@
 Ai
+Birnbaum
 CCF
 Fi
+Fussel
 LAnguage
 Napierian
 PSA
 REgular
 Ri
 Schematron
+Vesely
 datatype
 datatypes
 formulae