vcf-statistics

name	vcf-statistics
description	Generate variant statistics, sample concordance, and quality metrics using bcftools stats and gtcheck. Use when evaluating variant quality, comparing samples, or summarizing VCF contents.
tool_type	cli
primary_tool	bcftools

Version Compatibility

Reference examples tested with: bcftools 1.19+, numpy 1.26+

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show <package> then help(module.function) to check signatures
• CLI: <tool> --version then <tool> --help to confirm flags

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

VCF Statistics

Generate statistics and quality metrics using bcftools.

Statistics Tools

Command	Purpose
bcftools stats	Comprehensive variant statistics
bcftools gtcheck	Sample concordance and relatedness
bcftools query	Custom summaries

bcftools stats

Goal: Generate comprehensive variant statistics including counts, Ti/Tv ratio, and quality distributions.

Approach: Run bcftools stats and parse section-tagged output lines (SN, TSTV, AF, QUAL, DP).

"How many variants are in this VCF?" -> Compute summary counts, substitution types, and quality distributions from variant records.

Basic Statistics

bcftools stats input.vcf.gz > stats.txt

View Key Metrics

bcftools stats input.vcf.gz | grep "^SN"

Output sections:

• SN - Summary numbers
• TSTV - Transitions/transversions
• SiS - Singleton stats
• AF - Allele frequency distribution
• QUAL - Quality distribution
• IDD - Indel distribution
• ST - Substitution types
• DP - Depth distribution

Summary Numbers (SN)

bcftools stats input.vcf.gz | grep "^SN" | cut -f3-

Reports:

• Number of samples
• Number of records
• Number of SNPs
• Number of indels
• Number of multiallelic sites
• Number of multiallelic SNPs

Transition/Transversion Ratio

bcftools stats input.vcf.gz | grep "^TSTV"

Transitions (purine-to-purine: A↔G, or pyrimidine-to-pyrimidine: C↔T) are chemically favored over transversions because they preserve the purine/pyrimidine ring structure. CpG deamination (methylated C->T) is the single most common point mutation in vertebrate genomes and is a transition, which further inflates the Ti/Tv ratio. Exomes have higher Ti/Tv than whole genomes because coding regions are enriched for CpG dinucleotides.

Expected Ti/Tv ratio:

• Whole genome: ~2.0-2.1
• Exome: ~2.8-3.3
• Ti/Tv below expected range suggests excess false-positive SNPs (random errors produce Ti/Tv ~0.5)
• Ti/Tv above expected range suggests over-filtering that disproportionately removes transversions

Per-Sample Statistics

bcftools stats -s - input.vcf.gz > per_sample.txt

Compare Two VCFs

bcftools stats input1.vcf.gz input2.vcf.gz > comparison.txt

Region-Specific Stats

bcftools stats -r chr1:1000000-2000000 input.vcf.gz > region_stats.txt
bcftools stats -R exome.bed input.vcf.gz > exome_stats.txt

Plotting Statistics

Goal: Visualize variant statistics as publication-quality plots.

Approach: Pipe bcftools stats output to plot-vcfstats to generate PDF and PNG plots.

Generate Plots

bcftools stats input.vcf.gz > stats.txt
plot-vcfstats -p output_dir stats.txt

Creates:

• output_dir/summary.pdf
• Individual PNG files

Comparison Plots

bcftools stats file1.vcf.gz file2.vcf.gz > comparison.txt
plot-vcfstats -p comparison_dir comparison.txt

Expected QC Metric Ranges

Interpreting variant statistics requires context-dependent thresholds. A metric that looks normal in WGS may be alarming in WES.

Metric	WGS Expected	WES Expected	Flag If
Ti/Tv ratio	~2.0-2.1	~2.8-3.3	WGS: <1.8 or >2.5; WES: <2.5 or >3.5
Het/Hom ratio	1.5-2.0	1.5-2.0	Outlier relative to cohort
Call rate per variant	>95%	>95%	<90%
Call rate per sample	>95%	>95%	<90%
Singleton rate (WGS)	~100-200k	Variable	>2x cohort mean
Mendelian error rate (trios)	<0.5%	<0.5%	>1%

Interpretation notes:

• Het/Hom ratio too high suggests contamination (mixed DNA inflates heterozygosity); too low suggests inbreeding, population structure, or excess homozygous reference calls from low coverage
• Excess singletons per sample suggest sample quality issues, contamination, or library artifacts
• Call rate thresholds apply to common variants; rare variants naturally have higher missingness
• These ranges assume human diploid germline calling; somatic, polyploid, or non-model organisms require different expectations

Stratified Evaluation

A variant caller with 99% overall accuracy may perform at 70% in difficult genomic regions. Always evaluate stratified by region complexity.

bcftools stats -R easy_regions.bed input.vcf.gz > easy_stats.txt
bcftools stats -R difficult_regions.bed input.vcf.gz > difficult_stats.txt

Key stratification categories (using GIAB stratification BED files):

• Easy/high-confidence regions - baseline accuracy measurement
• Homopolymer runs - systematic indel errors, especially for Illumina and Ion Torrent
• Tandem repeats / low-complexity - alignment ambiguity inflates both FP and FN
• Segmental duplications - paralogous mapping produces false heterozygous calls
• High GC (>70%) / Low GC (<25%) - coverage bias creates systematic missingness
• MHC / centromeric regions - extreme polymorphism or repetitiveness defeats standard callers

GIAB stratification BED files are available at github.com/genome-in-a-bottle/genome-stratifications. Comparing Ti/Tv ratio between easy and difficult regions is a quick diagnostic: a drop in Ti/Tv within difficult regions confirms elevated false-positive rates there.

Population-Scale QC Metrics

For multi-sample VCFs, per-sample and per-site population metrics reveal systematic issues invisible in single-sample analyses.

Per-sample checks:

• Variant count should be roughly consistent across samples from the same population (~4.5M SNPs per WGS sample in humans); outliers suggest processing errors or contamination
• Per-sample Het/Hom ratio: outliers suggest contamination (high) or sample swaps between populations (unexpected value)
• Excess singletons in one sample relative to the cohort mean suggests library or sequencing issues

Per-site checks:

• Excess heterozygosity (InbreedingCoeff < -0.3 in GATK, or HWE test): suggests genotyping error at sites where reads from paralogous regions pile up
• Hardy-Weinberg equilibrium: sites failing HWE (p < 1e-5) within a homogeneous population may be genotyping artifacts. HWE filtering must be applied within populations, never across mixed-ancestry cohorts where true allele frequency differences violate HWE assumptions

bcftools query -f '%CHROM\t%POS\t%INFO/InbreedingCoeff\n' input.vcf.gz | \
    awk '$3 < -0.3' > excess_het_sites.txt

bcftools gtcheck

Goal: Verify sample identity and detect sample swaps by comparing genotype concordance.

Approach: Use bcftools gtcheck to compute pairwise discordance rates; interpret thresholds based on expected relatedness.

bcftools gtcheck -G 1 input.vcf.gz > relatedness.txt
bcftools gtcheck -g reference.vcf.gz query.vcf.gz

Discordance thresholds for interpreting results:

• Same individual (replicates/re-extractions): <0.5% discordance
• First-degree relatives (parent-child, siblings): ~10-15% discordance
• Unrelated individuals: >10% discordance (typically 15-25%)
• For trios: Mendelian error rate should be <0.5%; rates >1% suggest sample swaps or contamination

Output format (DC lines):

DC  0  sample1  sample2  0.95  1234  1200

Fields: DC tag, index, sample1, sample2, discordance rate, sites compared, discordant sites. Sample swaps are one of the most common errors in genomics studies. Running gtcheck is essential for any multi-sample study and should be performed early in the QC pipeline before downstream analysis.

Quick Statistics with Query

Goal: Compute targeted summary statistics using bcftools query and shell tools.

Approach: Extract specific fields with bcftools query/view and aggregate with awk for counts, means, and distributions.

Count Variants

bcftools view -H input.vcf.gz | wc -l

Count by Type

# SNPs
bcftools view -v snps -H input.vcf.gz | wc -l

# Indels
bcftools view -v indels -H input.vcf.gz | wc -l

Count PASS Variants

bcftools view -f PASS -H input.vcf.gz | wc -l

Quality Distribution

bcftools query -f '%QUAL\n' input.vcf.gz | \
    awk '{sum+=$1; count++} END {print "Mean QUAL:", sum/count}'

Depth Distribution

bcftools query -f '%INFO/DP\n' input.vcf.gz | \
    awk '{sum+=$1; count++} END {print "Mean DP:", sum/count}'

Genotype Counts

# Count heterozygous sites per sample
bcftools query -f '[%GT\t]\n' input.vcf.gz | \
    awk -F'\t' '{for(i=1;i<=NF;i++) if($i=="0/1" || $i=="0|1") het[i]++}
        END {for(i in het) print "Sample", i, "het:", het[i]}'

Allele Frequency Spectrum

bcftools query -f '%INFO/AF\n' input.vcf.gz | \
    awk '{
        if($1<0.01) rare++
        else if($1<0.05) low++
        else if($1<0.5) common++
        else freq++
    } END {
        print "Rare (<1%):", rare
        print "Low (1-5%):", low
        print "Common (5-50%):", common
        print "Frequent (>50%):", freq
    }'

Sample Statistics

Goal: Compute per-sample variant counts, genotype distributions, and missingness rates.

Approach: Use bcftools query/view/stats per sample to tabulate sample-level metrics.

List and Count Samples

bcftools query -l input.vcf.gz
bcftools query -l input.vcf.gz | wc -l

Per-Sample Variant Counts

for sample in $(bcftools query -l input.vcf.gz); do
    count=$(bcftools view -s "$sample" -H input.vcf.gz | \
        bcftools view -c 1 -H | wc -l)
    echo "$sample: $count"
done

Missing Genotypes per Sample

bcftools stats -s - input.vcf.gz | grep "^PSC"

cyvcf2 Statistics

Goal: Compute variant statistics programmatically in Python for custom analyses.

Approach: Iterate variants with cyvcf2, accumulate counts by type, and compute quality/genotype distributions with numpy.

Basic Counts

from cyvcf2 import VCF

stats = {'snps': 0, 'indels': 0, 'other': 0}

for variant in VCF('input.vcf.gz'):
    if variant.is_snp:
        stats['snps'] += 1
    elif variant.is_indel:
        stats['indels'] += 1
    else:
        stats['other'] += 1

print(f'SNPs: {stats["snps"]}')
print(f'Indels: {stats["indels"]}')
print(f'Other: {stats["other"]}')

Quality Statistics

from cyvcf2 import VCF
import numpy as np

quals = []
for variant in VCF('input.vcf.gz'):
    if variant.QUAL:
        quals.append(variant.QUAL)

quals = np.array(quals)
print(f'Mean QUAL: {np.mean(quals):.1f}')
print(f'Median QUAL: {np.median(quals):.1f}')
print(f'Min QUAL: {np.min(quals):.1f}')
print(f'Max QUAL: {np.max(quals):.1f}')

Genotype Distribution

from cyvcf2 import VCF

vcf = VCF('input.vcf.gz')
samples = vcf.samples

hom_ref = [0] * len(samples)
het = [0] * len(samples)
hom_alt = [0] * len(samples)
missing = [0] * len(samples)

for variant in vcf:
    for i, gt in enumerate(variant.gt_types):
        if gt == 0:
            hom_ref[i] += 1
        elif gt == 1:
            het[i] += 1
        elif gt == 3:
            hom_alt[i] += 1
        else:
            missing[i] += 1

for i, sample in enumerate(samples):
    print(f'{sample}: HOM_REF={hom_ref[i]}, HET={het[i]}, HOM_ALT={hom_alt[i]}, MISS={missing[i]}')

Transition/Transversion Calculation

from cyvcf2 import VCF

transitions = 0
transversions = 0

ti_pairs = {('A', 'G'), ('G', 'A'), ('C', 'T'), ('T', 'C')}

for variant in VCF('input.vcf.gz'):
    if not variant.is_snp:
        continue
    ref = variant.REF
    alt = variant.ALT[0]
    if (ref, alt) in ti_pairs:
        transitions += 1
    else:
        transversions += 1

ratio = transitions / transversions if transversions > 0 else 0
print(f'Transitions: {transitions}')
print(f'Transversions: {transversions}')
print(f'Ti/Tv ratio: {ratio:.2f}')

Common Workflows

Quality Control Report

bcftools stats input.vcf.gz > stats.txt
grep "^SN" stats.txt | cut -f3-
grep "^TSTV" stats.txt | cut -f5
plot-vcfstats -p qc_plots stats.txt

Compare Before/After Filtering

bcftools stats raw.vcf.gz filtered.vcf.gz > comparison.txt
grep "^SN" comparison.txt | head -20

Quick Reference

Task	Command
Full stats	bcftools stats input.vcf.gz
Summary only	bcftools stats input.vcf.gz | grep "^SN"
Ti/Tv ratio	bcftools stats input.vcf.gz | grep "^TSTV"
Per-sample	bcftools stats -s - input.vcf.gz
Compare VCFs	bcftools stats file1.vcf.gz file2.vcf.gz
Sample check	bcftools gtcheck -G 1 input.vcf.gz
Plot stats	plot-vcfstats -p dir stats.txt

Common Errors

Error	Cause	Solution
No data	Empty VCF	Check if VCF has variants
plot-vcfstats not found	Not installed	Install with bcftools
Cannot open	Invalid VCF	Check file format

Related Skills

• variant-calling/vcf-basics - View and query VCF files
• variant-calling/filtering-best-practices - Evaluate filter impact on QC metrics
• variant-calling/vcf-manipulation - Compare and merge call sets
• variant-calling/variant-calling - Assess calling quality
• variant-calling/joint-calling - Cohort-level genotyping where population metrics apply
• alignment-files/bam-statistics - Upstream alignment QC that affects variant statistics