diff --git a/lib/helpers/cpu.js b/lib/helpers/cpu.js
index 9ace6c1..50c68cd 100644
--- a/lib/helpers/cpu.js
+++ b/lib/helpers/cpu.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const os = require('os')
 const fs = require('fs')
 const exec = require('child_process').exec
diff --git a/lib/helpers/parallel.js b/lib/helpers/parallel.js
index 5fe90ec..1451e87 100644
--- a/lib/helpers/parallel.js
+++ b/lib/helpers/parallel.js
@@ -1,3 +1,5 @@
+'use strict'
+
 // execute an array of asynchronous functions in parallel
 // @param {Array} fns - an array of functions
 // @param {Function} done - callback(err, results)
diff --git a/lib/history.js b/lib/history.js
index b6085e3..b208cd6 100644
--- a/lib/history.js
+++ b/lib/history.js
@@ -1,4 +1,5 @@
 'use strict'
+
 const DEFAULT_MAXAGE = 60000
 
 const expiration = {}
diff --git a/lib/procfile.js b/lib/procfile.js
index ea069a7..3ebe812 100644
--- a/lib/procfile.js
+++ b/lib/procfile.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const fs = require('fs')
 const path = require('path')
 const updateCpu = require('./helpers/cpu')
diff --git a/test/bench.js b/test/bench.js
index b9a11e5..faf6805 100644
--- a/test/bench.js
+++ b/test/bench.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { spawn } = require('child_process')
 const test = require('ava')
 const tspan = require('time-span')
diff --git a/test/fixtures/_eventloop.js b/test/fixtures/_eventloop.js
index f9fa784..690a045 100644
--- a/test/fixtures/_eventloop.js
+++ b/test/fixtures/_eventloop.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const pidusage = require('../../')
 
 pidusage(process.pid, { maxage: 1500 }, function (err, stat) {
diff --git a/test/helpers/_mocks.js b/test/helpers/_mocks.js
index 3896c82..a08dde1 100644
--- a/test/helpers/_mocks.js
+++ b/test/helpers/_mocks.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const EventEmitter = require('events')
 const streamify = require('string-to-stream')
 const through = require('through')
diff --git a/test/integration.js b/test/integration.js
index cebab56..7b2ea4f 100644
--- a/test/integration.js
+++ b/test/integration.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { spawn } = require('child_process')
 const test = require('ava')
 const os = require('os')
diff --git a/test/procfile.js b/test/procfile.js
index 6cfc296..502adef 100644
--- a/test/procfile.js
+++ b/test/procfile.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const mockery = require('mockery')
 const test = require('ava')
 const os = require('os')
diff --git a/test/ps.js b/test/ps.js
index edbf477..4b091a1 100644
--- a/test/ps.js
+++ b/test/ps.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const mockery = require('mockery')
 const test = require('ava')
 const os = require('os')
diff --git a/test/wmic.js b/test/wmic.js
index b1bce08..518836e 100644
--- a/test/wmic.js
+++ b/test/wmic.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const mockery = require('mockery')
 const test = require('ava')
 const os = require('os')