Unemployment is double what it was a year ago.

分析：double 为限定词，what 融合型关系代词，what 引导的是表语从句，it 指代Unemployment。

句意：失业率是一年前的两倍。

解答:以上结构属于“A be+ times/twice/double + what 从句”结构用法

The length of the road is four times what it was three years ago.

这条路的长度是三年前的 4 倍。

The price of the meat is twice what it was last year.

肉价是去年的两倍。

The price of the cotton is double what it was five years ago.

棉花的价格是五年前的两倍。

类似倍数表达还有以下结构，供网友参考

【一】用 times 表示倍数（一般限于包括基数在内三倍或三倍以上的数。表示两倍的数，一般用 twice ）。其句式有：

【1】 “A is+数词 times ＋形容词 / 副词的比较级＋ than B” “A是B的几倍大”

Line AB is three times longer than line CD. 线段 AB 是（线段） CD 的三倍长。

This hall is five times bigger than our classroom. 这个大厅是我们的教室 5 倍大。

The meeting room is three times bigger than our office.这间会议室是我们办公室的3倍大。

【2】“A is+数词 times +as + 形容词 / 副词的原级＋ as +B” “A是B的几倍大”

This table is three times as long as that one. 这张桌子是那张桌子的三倍长。

This dictionary is five times as thick as the one you borrowed from the library.

这本词典的厚度是你从图书馆借的那本（厚度）的 5 倍。

This box is three times as heavy as that one.这个箱子是那个箱子的三倍重。

It cost three times as much as I had expected. 它花的钱比我预期的多两倍。

Cats sleep twice as much as people. 猫睡觉的时间比人长一倍。

Unemployment among blacks was twice as high as that for white.

黑人的失业人数是白人的两倍。

New drivers have twice as many accidents as experienced drivers.

新驾驶人所出的事故是老驾驶人的两倍。

【3】 “A is+ times + the + 名词（如： size， height， weight， length， width 等）＋ of …”

The earth is 49 times the size of the moon. 地球的体积是月球的 49 倍。

This river is three times the depth of that one. 这条河是那条河的三倍深。

This street is four times the length of that one.这条街是那条街道的四倍长。

Their house is twice the size of ours. 他们的房子是我们的两倍大。

=Their house is twice as large as ours.

The queen bee is twice the length of a worker bee. 蜂王的体长为工蜂的两倍。

This room is twice the length of the other, but much narrower.

这个房间的长度是那个房间的两倍，但窄得多。

Concorde crosses the Atlantic at twice the speed of sound.

协和飞机以两倍于声速的速度飞越大西洋。

He eats twice the amount that you eat.= He eats twice what you eat. 他比你多吃一倍。

This box is twice the size of that one.= This box is twice as large as that one.

这个盒子是那个盒子的两倍。

【4】“… times + more ＋名词＋ than …”

He earns five times more money than he did ten years ago.

他现在挣的钱比十年前挣的多 5 倍。

There are twice more students in our class than in theirs.

我们班的学生人数比他们班多两倍。

【5】“… times +as many （或 much ）＋名词＋ as …”

We've produced twice as much cotton this year as （ we did ） ten years ago.

今年我们生产的棉花比十年前多了一倍。（汉语中的多一倍实际上也指两倍一样多。）

He has got three times as many books as his sister.

他拥有的书的数量是他妹妹的 3 倍。

【6】“… times + what 从句”

The length of the road is four times what it was three years ago.

这条路的长度是三年前的 4 倍。

The price of the meat is twice what it was last year.

肉价是去年的两倍。

【7】  “… times + up on （或 over ）…”

The size of the sun is a million times over that of the earth.

太阳的体积比地球的体积大一百万倍。

He is three times up on your age. 他的年龄是你的4倍（或：比你的年龄大3倍）。

【8】“… +by + … times”

Compared with last year， our coal output has increased by three times.

与去年相比，我们的煤产量增长了3倍。

His speed is faster than mine by four times. 他的速度比我的快 4 倍。

【9】“… times + compared with ＋被比较对象”

The number of the students in our school has increased for times compared with 1980.

我们学校的学生人数比 1980 年增加了3倍。

【10】“… times + that of + 被比较对象”

In this workshop the output of July was 3.5 times that of January.

这个车间七月份的产量是元月份的 3.5 倍。