ANCOM results: phyla with W values of 0 identified as differentially abundant

[smd (Sara Dias)]

Hi,
I'm analyzing a set of samples of COPD patients and Healthy controls, and I've performed ANCOM to assess the differential abundant phyla between the two groups.
These are the results that I've obtained.
|k__Bacteria;p__Proteobacteria |3|
|k__Bacteria;p__Spirochaetes |2|
|k__Bacteria;p__Firmicutes |1|
|k__Bacteria;p__Synergistetes |1|
|k__Bacteria;p__Verrucomicrobia |1|
|k__Bacteria;p__Actinobacteria |0|
|k__Bacteria;p__Bacteroidetes |0|
|k__Bacteria;p__Chloroflexi |0|
|k__Bacteria;p__Fusobacteria |0|
|k__Bacteria;p__GN02 |0|
|k__Bacteria;p__SR1 |0|
|k__Bacteria;p__TM7 |0|
|k__Bacteria;p__Tenericutes |0|

Can you give me a hint on how to interpret these zero w values?
thank you in advance
best regards
Sara

[jwdebelius (Justine Debelius)]

Hi @smd,

ANCOM looks at the ratio between features as the input for a test, and the W statistic is the number of ratios between that feature and every other that was significantly different. So, an ANCOM W of 0 means that no tests were significantly different.

Your results at the phylum level aren’t terribly surprising; Phylum level is a really high level grouping and many people find higher level differences are driven by changes at lower levels (often ASV/OTU or genus).

Best,
Justine