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

Basic Statistics

bash

bcftools stats input.vcf.gz > stats.txt

View Key Metrics

bash

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)

bash

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

bash

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

Expected Ti/Tv ratio:

•Whole genome: ~2.0-2.1
•Exome: ~2.8-3.3

Per-Sample Statistics

bash

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

Compare Two VCFs

bash

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

Region-Specific Stats

bash

bcftools stats -r chr1:1000000-2000000 input.vcf.gz > region_stats.txt

Exome Statistics

bash

bcftools stats -R exome.bed input.vcf.gz > exome_stats.txt

Plotting Statistics

Generate Plots

bash

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

Creates:

•output_dir/summary.pdf
•Individual PNG files

Comparison Plots

bash

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

bcftools gtcheck

Check Sample Identity

bash

bcftools gtcheck -g reference.vcf.gz query.vcf.gz

Reports concordance between samples.

Detect Sample Swaps

bash

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

Compares all samples pairwise.

Output Format

code

DC  0  sample1  sample2  0.95  1234  1200

Fields:

•DC: Data type (discordance)
•Index
•Sample 1
•Sample 2
•Discordance rate
•Sites compared
•Discordant sites

Check Against Reference Panel

bash

bcftools gtcheck -g 1000genomes.vcf.gz unknown_sample.vcf.gz

Quick Statistics with Query

Count Variants

bash

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

Count by Type

bash

# 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

bash

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

Quality Distribution

bash

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

Depth Distribution

bash

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

Genotype Counts

bash

# 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

bash

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

List Samples

bash

bcftools query -l input.vcf.gz

Count Samples

bash

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

Per-Sample Variant Counts

bash

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

bash

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

cyvcf2 Statistics

Basic Counts

python

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

python

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

python

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

python

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

bash

# Generate stats
bcftools stats input.vcf.gz > stats.txt

# Extract key metrics
echo "=== VCF Summary ==="
grep "^SN" stats.txt | cut -f3-

echo ""
echo "=== Ti/Tv Ratio ==="
grep "^TSTV" stats.txt | cut -f5

# Generate plots
plot-vcfstats -p qc_plots stats.txt

Compare Before/After Filtering

bash

bcftools stats raw.vcf.gz filtered.vcf.gz > comparison.txt

echo "=== Before Filtering ==="
grep "^SN.*raw" comparison.txt | cut -f3-

echo ""
echo "=== After Filtering ==="
grep "^SN.*filtered" comparison.txt | cut -f3-

Sample Relatedness Check

bash

bcftools gtcheck -G 1 cohort.vcf.gz > relatedness.txt
cat relatedness.txt

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

•vcf-basics - View and query VCF files
•filtering-best-practices - Evaluate filter impact
•vcf-manipulation - Compare call sets
•variant-calling - Assess calling quality