diff --git a/lib/App/Sandy/Error.pm b/lib/App/Sandy/Error.pm
new file mode 100644
index 00000000..9b72b7f3
--- /dev/null
+++ b/lib/App/Sandy/Error.pm
@@ -0,0 +1,32 @@
+package App::Sandy::Error;
+# ABSTRACT: Base class for error models
+
+use App::Sandy::Base 'class';
+
+# VERSION
+
+has '_not_base' => (
+	is         => 'ro',
+	isa        => 'HashRef',
+	builder    => '_build_not_base',
+	lazy_build => 1
+);
+
+sub _build_not_base {
+	my %not_base = (
+		A => ['T', 'C', 'G'],
+		a => ['t', 'c', 'g'],
+		T => ['A', 'C', 'G'],
+		t => ['a', 'c', 'g'],
+		C => ['A', 'T', 'G'],
+		c => ['a', 't', 'g'],
+		G => ['A', 'T', 'C'],
+		g => ['a', 't', 'c']
+	);
+	return \%not_base;
+}
+
+sub randb {
+	my ($self, $base, $rng) = @_;
+	return $self->_not_base->{$base}[$rng->get_n(3)] || $base;
+}
diff --git a/lib/App/Sandy/Error/Phred.pm b/lib/App/Sandy/Error/Phred.pm
new file mode 100644
index 00000000..61285c0d
--- /dev/null
+++ b/lib/App/Sandy/Error/Phred.pm
@@ -0,0 +1,37 @@
+package App::Sandy::Error::Phred;
+# ABSTRACT: Calculate error based on the phred score
+
+use App::Sandy::Base 'class';
+
+use constant ASCII_INIT => 0x21;
+
+extends 'App::Sandy::Error';
+
+# VERSION
+
+sub insert_sequencing_error {
+	my ($self, $read_ref, $quality_ref, $read_size, $rng) = @_;
+	my @errors;
+
+	for (my $i = 0; $i < $read_size; $i++) {
+		my $char = substr $$quality_ref, $i, 1;
+
+		if ($self->_is_there_any_error($char, $rng)) {
+			my $base = substr $$read_ref, $i, 1;
+			my $not_base = $self->randb($base, $rng);
+
+			substr($$read_ref, $i, 1) = $not_base;
+
+			push @errors => sprintf("%d:%s/%s", $i + 1, $base, $not_base);
+		}
+	}
+
+	return \@errors;
+}
+
+sub _is_there_any_error {
+	my ($self, $char, $rng) = @_;
+	my $score = ord($char) - ASCII_INIT;
+	my $prob = 10 ** (-$score / 10);
+	return int($rng->uniform() * (1 / $prob)) == 0;
+}
diff --git a/t/lib/TestsFor/App/Sandy/Error.pm b/t/lib/TestsFor/App/Sandy/Error.pm
new file mode 100644
index 00000000..b88121fa
--- /dev/null
+++ b/t/lib/TestsFor/App/Sandy/Error.pm
@@ -0,0 +1,45 @@
+package TestsFor::App::Sandy::Error;
+# ABSTRACT: Tests for 'App::Sandy::Error' class
+
+use App::Sandy::Base 'test';
+use App::Sandy::RNG;
+
+use autodie;
+use base 'TestsFor';
+
+use constant SEED => 17;
+
+sub startup : Tests(startup) {
+	my $test = shift;
+	$test->SUPER::startup;
+	my $class = ref $test;
+	$class->mk_classdata('default_error');
+	$class->mk_classdata('rng');
+}
+
+sub setup : Tests(setup) {
+	my $test = shift;
+	my %child_arg = @_;
+	$test->SUPER::setup;
+
+	$test->default_error($test->class_to_test->new());
+	$test->rng(App::Sandy::RNG->new(SEED));
+}
+
+sub not_base : Test(16) {
+	my $test = shift;
+
+	my $class = $test->class_to_test;
+	my $error = $test->default_error;
+	my $rng = $test->rng;
+
+	my @bases = (qw/A T C G a t c g/);
+
+	for my $base (@bases) {
+		my $not_base = $error->randb($base, $rng);
+		isnt $base, $not_base,
+			"base '$base' should not be equal ~base '$not_base'";
+		like $not_base, qr/[ATCGatcg]/,
+			"~base '$not_base' should be in [ATCGatcg]";
+	}
+}
diff --git a/t/lib/TestsFor/App/Sandy/Error/Phred.pm b/t/lib/TestsFor/App/Sandy/Error/Phred.pm
new file mode 100644
index 00000000..02dbac5f
--- /dev/null
+++ b/t/lib/TestsFor/App/Sandy/Error/Phred.pm
@@ -0,0 +1,57 @@
+package TestsFor::App::Sandy::Error::Phred;
+# ABSTRACT: Tests for 'App::Sandy::Error::Phred' class
+
+use App::Sandy::Base 'test';
+use App::Sandy::RNG;
+use base 'TestsFor::App::Sandy::Error';
+
+use autodie;
+use base 'TestsFor';
+
+sub startup : Tests(startup) {
+	my $test = shift;
+	$test->SUPER::startup;
+}
+
+sub setup : Tests(setup) {
+	my $test = shift;
+	my %child_arg = @_;
+	$test->SUPER::setup;
+
+	$test->default_error($test->class_to_test->new());
+}
+
+sub insert_error : Tests(3) {
+	my $test = shift;
+
+	my $class = $test->class_to_test;
+	my $error = $test->default_error;
+	my $rng = $test->rng;
+
+	my $read = 'ATCG' x 10;
+	my %dep = (
+		'!' => {
+			min => 39, max => 41,
+			msg => sub{ "n_errors '$_[0]' should be between ]39,41[" }
+		},
+		'$' => {
+			min => 14, max => 26,
+			msg => sub{ "n_errors '$_[0]' should be between ]14,26[" }
+		},
+		'%' => {
+			min => 10, max => 23,
+			msg => sub{ "n_errors '$_[0]' should be between ]10,23[" }
+		}
+	);
+
+	for my $char (keys %dep) {
+		my $quality = $char x 40;
+
+		my $errors_a = $error->insert_sequencing_error(\$read,
+			\$quality, 40, $rng);
+		my $n_errors = scalar @$errors_a;
+
+		ok $n_errors < $dep{$char}{max} && $n_errors > $dep{$char}{min},
+			$dep{$char}{msg}->($n_errors);
+	}
+}