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+This directory contains a set of test files in the CRAM 3.1 format.
+
+As CRAM 3.1 is identical to CRAM 3.0 except the version number and
+choice of codecs available, this directory does not contain a large
+number of files.  It is recommended that CRAM 3.1 implementations
+first validate themselves against the CRAM 3.0 data sets, along with
+validating the specific codecs in isolation.
+
+Once those two checks have been performed, the files here act as an
+integration test for the full CRAM 3.1 specification.
+
+The 4 CRAM files use successive levels of compression codecs in
+use.  These are purely representative of choices that could be made in
+trading encode time, decode time, granularity of random access and
+file size.  They are not prescriptive or a requirement for any encoder
+to follow and are intended as examples only.
+
+There are analogous CRAM v3.0 files in cram/3.0/passed/level-[124].cram
+and BAM at various gzip levels in bam/passed/level-[19].bam.
+All of these files have identical content, with just the encoding
+methods differing.
+
+The `samtools cram-size -v` tool can be used for reporting the codecs
+used within each CRAM file.
+
+### level-1.cram
+
+Deflate (gzip) level 1, rANS Nx16 order-0 and order-1 only.  The slice
+size here is also small.  Some small data blocks are uncompressed.
+An example file designed for fine-grained random access and fast decoding.
+
+### level-2.cram
+
+As level-1, but with higher levels of Deflate compression, adds use of
+the name tokeniser, and extra data transforms used in the rANS Nx16
+codec.  The number of sequences per slice is higher too, offering
+a more balanced size versus random access granularity.
+
+### level-3.cram
+
+Uses bzip2 and fqzcomp in addition to the above.  Also uses a
+larger container / slice size.  This is more appropriate for long-term
+archival purposes.
+
+### level-4.cram
+
+Maximum compression with LZMA and the range-coder in use, plus more
+aggressive compression of previous codecs.
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